Water and salt balance was studied in the same goats during pregnancy, lactation and anestrus. All goats increased their water intake during the course of pregnancy, but individual differences were large. In general, twin pregnant (TP) goats drank more water than single pregnant (SP) animals (3.4 +/- 0.4; N = 11 as compared to 2.5 +/- 0.3 litres/day; N = 5) during the last weeks of pregnancy. During lactation the high water intake TP goats persisted and the SP animals increased their intake to the same level as the TP goats. The water intake was reduced to about 2 1/day in both categories of animals during anestrus. Urine volume largely followed the changes in water intake in the individual animal. A continuous decrease in urine osmolality during the course of pregnancy occurred, but during lactation urine osmolality increased towards anestrus levels. TP goats generally retained more sodium than SP animals during pregnancy and during anestrus, whereas the figures were similar during lactation. Plasma Na, K and osmolality remained unchanged during pregnancy, lactation and anestrus, but a large fall in total plasma proteins and a moderate fall in hematocrit were observed during the course of pregnancy. Glomerular filtration rate of TP goats was elevated by about 35% during the 4th month of pregnancy, but did not differ from anestrus levels during the 3rd and 5th month or during lactation. Effective renal plasma flow was highest during the 3rd pregnancy month and then fell to reach lactation and anestrus levels during the 4th month of pregnancy. A few hours before parturition the animals became markedly dehydrated as shown by sudden increases in plasma Na, K, osmolality, total proteins and hematocrit. This water deficit was replenished within 26 h post-partum.