The relative importance of thermal, osmotic and hypovolaemic factors in the control of drinking in the pig.

1. The influence of thermogenic, osmotic or hypovolaemic stimuli on drinking has been measured in young pigs. Temperature changes in the hypothalamus and spinal cord were induced by means of implanted thermodes, while peripheral changes in skin and scrotal temperature were induced by circulating water through a coat. Osmotic stimuli were applied by I.V. injections of NaCl and hypovolaemia was induced by withdrawal of blood. 2. At an ambient temperature of 21--24 degrees C warming the hypothalamus for 10 min increased the water intake approximately 40%, while cooling the hypothalamus for a similar period reduced it by about 70%. 3. At an ambient temperature of 21--24 degrees C warming the scrotum caused the pigs to drink more than controls, and at the higher ambient temperature of 32 degrees C the effect was even greater. 4. Warming or cooling the spinal cord produced effects that were not consistent either within or between animals, thus temperature changes in this region play, at most, a minimal physiological role in the control of drinking in the pig. Warming the skin to 40 degrees C produced a slight increase in water intake, as also did cooling of the skin. 5. Drinking induced by an osmotic stimulus was proportional to the concentration of NaCl ranging from 5 to 30%. 6. Withdrawal of 500 ml. blood did not produce a statistically significant increase in water intake compared with controls.