A two-pool model of tritiated water kinetics was used to estimate the major body water pools, and hence body composition, in goats at days 10, 38 and 76 of lactation. Between days 10 and 38 of lactation goats were, on average, in negative calculated energy balance and were estimated to have mobilized 59 g body fat stores/d. Mean calculated energy balance over days 38-76 of lactation was slightly positive and there was little change in estimated body fat. Gut fill increased over the early part of lactation when goats were mobilizing body fat. Consequently, live weight did not differ at any stage of lactation and did not provide a good index of body fat status of the goats. There were also no significant differences in empty-body-weight, water, protein, ash or fat-free mass at the three stages of lactation. As average calculated energy balance and changes in energy stored as fat were highly correlated, it is concluded that the two-pool model of tritiated water kinetics is a useful means of serially estimating changes in body fat content in unfasted lactating goats.