1. During 24 hr water deprivation, the pregnant and lactating rats lost more weight than the non-mated animals. The increase in haematocrit was considerably greater in the lactating animals although plasma osmolality increased to the same extent in both lactating and non-mated animals. 2. Following water deprivation, the water intake of the lactating rats was significantly less than that of the pregnant, post-lactating or non-mated animals when expressed as a percentage of the weight loss during dehydration. While the non-mated rats restored their plasma osmolality to the pre-deprivation value upon drinking, the lactating rats sustained a new decrease in osmolality despite the fact that their mean haematocrit was still elevated. 3. When the dehydrated animals were given isotonic saline to drink instead of water, both lactating and non-mated animals restored their body weight and haematocrit to pre-deprivation levels. 4. Lactating rats showed a reduced absolute and net fluid intake in response to I.V. hypertonic saline which could not be explained by more rapid renal clearance of the solute. 5. The pregnant and lactating rats drank less in response to s.c. isoprenaline than did the post-lactating and non-mated animals. 6. The spontaneous 3 hr fluid intake was significantly less during lactation. 7. It is suggested that during the hours of daylight, lactating rats are less responsive to dipsogenic stimuli arising from deficits in both their intracellular and extracellular fluid spaces.