Ten animals from two litters of English large white pigs were weaned at 12—15 days of age and placed one per cage within constant—temperature rooms. In the first group, a comparison was made between three animals raised at 5°C and their littermates raised at 35°C. There were marked differences in appearance and in gross morphology. Those from the cold environment had more hair, were shorter and more stocky, and had a shorter tail and smaller ears than their littermates. A second group of animals were raised at 20°C and 35°C; it was found that with respect to all measurements the 20°C pigs were intermediate between the 5° and 35°C animals. These observations accord with Bergmann's Rule that animals raised in colder environments are characterized by a reduced surface area/body weight ratio. The results likewise substantiate Allen's Rule that in colder climates the length of appendages and of extremities is reduced.

SUMNARY1. Pigs were trained to push a switch with their snouts in order to obtain a short burst of infra-red heat on the skin.2. When the preoptic region of the hypothalamus was cooled by means of an implanted thermode, the rate at which the heaters were turned on increased at environmental temperatures ranging from 0 to 2500. At 30 and 350 C cooling sometimes had no effect.3. The preoptic region was warmed either by means of a thermode or using radio-frequency heating from implanted electrodes. Warming the preoptic region decreased the rate at which the infra-red heaters were turned on, but the effect was not as obvious as the increases observed during cooling.4. Pigs placed in a cold or neutral environment did not learn to push a switch in order to obtain radio frequency heating of the preoptic region.

SUMMARY1. The rate of blood flow through the tail of the pig has been measured by means of venous occlusion plethysmography using a mercury in rubber strain gauge. Conscious animals were used in all experiments because previous work had demonstrated that anaesthetics interfere with the animal's ability to vasoconstrict.2. Graded changes in the temperature of the hypothalamus were imposed by means of an implanted thermode. It was found that the change in blood flow depended on the extent of the change in hypothalamic temperature and on the ambient temperature. Below 200 C ambient temperature, heating the hypothalamus did not cause vasodilatation and at 300 C ambient temperature, cooling the hypothalamus caused only slight vasoconstriction, but at 250 C ambient temperature, changes in hypothalamic temperature caused changes in blood flow from full vasoconstriction to full vasodilatation.3. The skin temperature on the trunk was changed by means of water circulated through tubes sewn into a coat worn by the pig. Blood flow in the tail, which was outside the coat, depended on the skin temperature of the trunk, the ambient temperature, and the temperature of the hypothalamus, all of which were varied separately. 4. A thermode was implanted in the epidural space in the cervical region of the spinal cord. The change in blood flow in the tail which accompanied a change in thermode temperature was found to depend on the temperature of the thermode and the ambient temperature. Cooling the spine while the hypothalamus was being heated to 430 C resulted in a decrease in blood flow, but when the spine was heated while the hypothalamus was being cooled the increase in blood flow was only slight.5. Local stimulus to the tail in the form of infra-red heat, or increased air movement was followed by changes in blood flow even when deep body temperature remained stable.

SUMMARY1. Plasma ADH concentration, urinary and plasma osmolality and -haematocrit were measured in young pigs placed in cold, thermoneutral, warm and hot ambient temperatures. In some experiments a thermode placed in the hypothalamus or over the cervical spinal cord was heated or cooled at various ambient temperatures.2. Plasma ADH concentration remained at a low level (0.5-5 ,uu. ml.-') over 2 hr or 3 hr periods when the pigs were in cold, thermoneutral or warm ambient temperatures. A hot environment, which caused a marked rise in the pigs' rectal temperature, was associated with a large rise in plasma ADH level.3. The rise in plasma ADH level which occurred during an increase in body temperature was consistently and completely suppressed by simultaneous cooling of the thermode in the pre-optic region to 5 or 10°C. When the thermode was in the region of the supraoptic nucleus the rise in ADH was only partly suppressed, and when it was over the cervical cord it was only sometimes suppressed. 4. Cooling the thermodes in any position at a cold or thermoneutral ambient temperature, or heating them at a thermoneutral or warm ambient temperature, caused no consistent change in ADH.5. A diuresis, with a urinary flow-rate of at most 1 ml. min-and minimal urinary osmolality of 53 m-osmole kg-', was observed on only three occasions, twice during cooling of a thermode in the hypothalamus and once after the end of a period when the thermode was heated. In each case, the plasma ADH was less than 2 /su. ml.-'.M. L. FORSLING AND OTHERS 6. A slight rise of haematocrit in cold ambient conditions and a slight fall in the warm were observed. Otherwise changes in haematocrit were trivial, and a shift of water between vascular system and interstitium could not be invoked to account for changes in ADH levels. Observed variation of plasma osmolality was also slight.

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