The response to 30 h water deprivation was studied in 7 goats during the last month of pregnancy and during lactation with anestrus as the control period. Plasma osmolality and plasma Na concentration increased by about 4% in pregnant and lactating goats and by about 2% in anestral goats. Plasma AVP concentration rose by about 7 pg/ml in pregnant and lactating goats, but only by about 3 pg/ml during anestrus. PRA was elevated in pregnant animals, but dehydration caused only a minor further rise. Total plasma protein concentration was low in pregnant goats and did not increase during water deprivation, but it did so in lactating animals. Neither the hematocrit nor the plasma K concentration changed in response to dehydration. GFR fell by about 24% in pregnant goats and by 22% in lactating animals, but remained unchanged during anestrus. ERPF fell by 20% in lactating animals, but no consistent effect of the dehydration was seen during pregnancy and anestrus. Urine flow decreased by about 75% during pregnancy, 55% during lactation and 65% during anestrus with the highest urine osmolality observed during anestrus. Milk production was only slightly reduced, but the milk osmolality increased in parallel with that of the blood plasma. When allowed to drink at the end of the water deprivation period, pregnant goats immediately drank 2.5 +/- 0.5 litres, lactating goats 3.3 +/- 0.9 litres and anestral goats 1.1 +/- 0.3 litres. When hyperhydrated, pregnant goats excreted the excessive water more readily and showed less response to exogenous AVP than lactating and anestral animals. In conclusion, pregnant and lactating goats are obviously more susceptible to a shortage of water supply than anestral animals but can easily excrete an excess of water.