The mechanism of drinking induced by parenteral hyperonocotic solutions in the pigeon and in the rat.

1. 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.