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Brown & Holmes (1956) have recently studied the after-effects of activity on the electrical responses of mammalian C fibres. The main effect which they found was that shortly after a brief period of repetitive stimulation there was an enhancement in the height of the recorded monophasic action potential, which sometimes increased threefold. The increase was attributed to an increase in the amplitude of the spike potential in each individual nerve fibre, and was thought not to be caused to any great extent by the increase in the size and extent of the slow after-potentials which also occurred, a possibility put forward by Gasser (1950). The enhancement of the recorded spike which was shown by Brown & Holmes to be characteristic of all groups of non-medullated fibres conducting at less than 1 m/sec, is best obtained under the right conditions of temperature and frequency and duration of stimulation; but even then the effect in nerves in vitro is often small or absent. The present experiments deal with the nature of the effect. METHODSAdult lop-eared rabbits were anaesthetized with urethane given as a 25 % solution into the marginal ear vein (1.6 g/kg). About 50 mm of the cervical sympathetic trunk was removed from the animal and cleaned under a high-power ( x 25) dissecting microscope; all connective tissue was removed and occasionally the trunk or one of its small subdivisions was desheathed. In a few experiments the hypogastric nerve was used instead of the cervical sympathetic trunk: the results obtained with this nerve were so similar to those obtained with the cervical sympathetic trunk that they will not be dealt with separately.In the earlier experiments most of the fibres of the desheathed nerve were cut away so that only a small bundle of fibres remainedwhose cross-section was about -I of the original area of the trunk. Such a bundle was mounted in the apparatus described by Stampfli (1954) and in more detail by Straub (1956), so that it lay in two pools of Locke's solution which were separated by a third insulating pool of isotonic sucrose solution. A pair of non-polarizable electrodes, dipped one * Fellow of the Swiss Academy of Medical Sciences.

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