Body water loss and turnover rate were measured in fasting newly weaned grey seal (Halichoerus grypus) pups without access to water for 52 days, by use of bolus injections of deuterated water. Total body water (N) was 41.8 +/- 2.3 (SD)% of total body weight at day 3 and 44.5 +/- 5.9 (SD)% at day 51 of fasting (P greater than 0.05), while the water content of body core decreased from 73 to 61% over the same period. The average water efflux rate in the same period was 362 +/- 17 (SD) ml d-1, the reduction in N over the same period contributing 35% to this end. The biological half-time of deuterated water was 38.2 +/- 3.3 (SD) d. Plasma osmolality increased from a stable level of 329 +/- 11 (SD) mosmol kg-1 to 445 +/- 11 (SD) mosmol kg-1 after day 38 of fasting. Plasma urea was the major contributor to this increase, in spite of a steadily decreasing urine urea concentration (Nordøy et al. 1990) throughout the entire experimental period. It is concluded that grey seal pups endure 52 d of fasting without intake of water with a minor dehydration of body core, while they become hyperosmotic due to increased catabolism of protein after 38 d when sparing of the insulating blubber layer in preparation for life in cold water seems to occur.