SUMMARY1. In nephrectomized rats, s.c. (0*12 mg.kg body wt.-') or intracerebroventricular (i.c.v.: 0-03 mg. kg-'), isoprenaline failed to elicit drinking. However, when preceded (5-20 min) by a non-dipsogenic dose of i.v. pig renin, s.c. isoprenaline induced a marked, and i.c.v. isoprenaline a smaller drinking response. 2 hr after i.v. renin, s.c. isoprenaline no longer caused drinking.2. Pig renin did not enhance drinking in response to 0*12 mg.kg-' isoprenaline s.c. in intact or sham-operated rats.3. Isoprenaline (0.12 mg.kg body wt.-', s.c.) caused a larger fall of blood pressure in unanaesthetized nephrectomized than in intact unanaesthetized rats, but it was not the resulting hypotension that interfered with the nephrectomized rats' ability to drink, since intact rats with similar falls in blood pressure drank avidly in response to large doses of isoprenaline.4. Since the rate of inactivation of pig renin in nephrectomized rats was not modified by isoprenaline, drinking in nephrectomized animals in response to renin + isoprenaline was not attributable to increased plasma renin levels.5. Since isoprenaline induces drinking in the presence of circulating renin, but in the absence of renin release from kidneys, renin plays a permissive role in isoprenaline-induced drinking. Angiotensin and isoprenaline may interact at the level of intracranial receptors.

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SUMMARY1. In pigeons, the i.v. injection of 0 3-1 3 ml. of 50% (w/w) solutions of either polyethylene glycol (mol.wt.. 20,000) or dextran (mol.wt. 40,000) induced reliable, rapid dose-dependent drinking responses. The amount of water drunk in response to i.v. polyethylene glycol was greater than that in response to i.P. polyethylene glycol and twice that in response to i.v. dextran. i.v. polyethylene glycol induced a diuresis following the onset of drinking.2. The dipsogenic effect of i.P. polyethylene glycol solutions in the pigeon was depressed by i.v. isotonic NaCl solution 1 hr before offering water but was increased by simultaneous i.v. hyperoncotic polyethylene glycol solution.3. In contrast, in rats, the subcutaneous injection of 25 % (w/w) polyethylene glycol (mol.wt. 20,000; 1-25-10-0 ml./kg body wt.) induced reliable drinking responses, while the same doses of polyethylene glycol, injected i.v., did not induce drinking consistently. The volume of water drunk by rats in response to subcutaneous polyethylene glycol was smaller per unit dose than that drunk by i.P. injected pigeons.4. The results suggest that receptors for drinking induced by extracellular dehydration in the pigeon could be situated in the extravascular interstitial section of the extracellular compartment.

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