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Rats consume significantly more of a solution combining saccharin and glucose than equivalent solutions of these substances presented in separate bottles. Fluid consumption may exceed body weight. Experiments helping to delineate the basis of this synergistic action are presented.

Direct chemical stimulation With carbachol in the hypothalamus of the rabbit caused a significant increase in food intake, as a function of place stimuilated and concentration. Lower doses of carbachol injected near the supraoptic nuleus produced an increase in drinking without an increase in eating.

1. Intravenous injection of five Goldblatt units (Gbl.U) of renin, or intravenous infusion of 2.5 mug of angiotensin into nephrectomized rats decreased plasma volume by 22 +/- 2% (calculated from the increase of the haematocrit) or by 14 +/- 2% (calculated from the increase in protein concentration) within 2 minutes. Fifteen minutes and 60 min after injection or infusion, the plasma volume (calculated from both parameters) was still 10% lower than initially.2. The initial discrepancy between haematocrit values and plasma protein concentration was neither due to mobilization of red blood cells (r.b.c.) from the spleen, nor to an increase in r.b.c. volume.3. The observed decrease in plasma volume was large enough to explain the drinking response to renin and angiotensin. Renin may thus induce or aggravate hypovolaemia rather than signalling its occurrence to hypothalamic receptors.