SUMMARY1. Acute changes in total body haematocrit were produced in anaesthetized dogs by the rapid infusion of packed red cells, dextran solution or hypertonic mannitol. A reduction of haematocrit resulted in a decrease in filtration fraction, and a rise of haematocrit in an increase in ifitration fraction. A direct relationship between water reabsorption (relative to filtered load) and packed cell volume (P.c.v.) was also observed.2. It is suggested that the changes in filtration fraction accompanying changes in haematocrit are mediated largely by a passive alteration of efferent arteriolar resistance resulting from modifications ofblood viscosity.

a logical approach. If an insulinoma is suspected a means of confirming the diagnosis would be to take three morning plasma samples after an overnight fast. In a subject who has fasting hypoglycaemia an amended ratio of more than 50 AU/ mg, in the absence of very severe obesity, is strongly suggestive of an insulinoma, and a ratio of more than 200 ,uU/mg would appear to be diagnostic. Very occasionally patients with extreme obesity, pregnancy, or liver disease with a portosystemic shunt might give "false-positive" results; in most doubtful cases an ethanol infusion could be performed as described elsewhere (Turner et al., 1971).It has recently been shown that insulinomas secrete a high proportion of proinsulin (Melani et al., 1970a). This does not invalidate the examination of fasting insulin levels as a diagnostic measure, as a high level of immunoreactive insulin relative to the plasma glucose is the diagnostic feature-any cross-reacting proinsulin from an insulinoma would exaggerate an abnormal ratio.

case ventilatory depression returns. The use of a larger dose of naloxone, given either intravenously or intramuscularly, is being investigated to determine whether the action of naloxone can be usefully prolonged.We thank the midwives, obstetricians, and paediatricians of the University Hospital of Wales for their help and co-operation in the study. There was no significant difference in the peak levels or pattern of enzyme increase. The incidence of cardiac failure and reinfarction was similar in the two groups, but major arrhythmias were less common in those on streptokinase (P <0 05). In the streptokinase group there were 36 minor and six more serious haemorrhagic com- References

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