Body composition data estimated for normal young, normal adult, and adult obese Wistar rats by a tritiated water (THO) dilution procedure (b. wt. range 119-237 g, n = 31; 305-597 g, n = 43; 274-381 g, n = 6) were compared with the results of whole body analysis. As for the predicted total body lipid (TBL), the content was calculated from the virtual THO-dilution space estimated by a simple micro-method and a regression equation between TBL [%] and DM [%] (dry matter) content (Part 1. Relations between directly measured major body components. Nahrung 35 (1991) 581. Due to different degrees of overestimations in the animal groups examined, the THO-space can be corrected via subtraction of 3.8%, 2.2% and 1.1% in the sequence young, obese, and adult groups. The derived TBL [%] data show a non-significant deviation of less than 1 g/100 g b. wt. in comparison to the three group means after direct lipid determinations. In the best case, i. e. after the additional elimination of extremely overestimated values for total body water (outliers) the combined tritium-derived lipid values (y) are correlated with the lipid data obtained analytically (x), both as proportion of the body weight, by the regression equation y = 0.848 + 0.963 x, r = 0.985, Sy.x = 1.77, n = 58. For a group of young rats (n = 14) after a pre-experimental supplementation with a sucrose solution, TBL values, not significantly different from analytically determined ones, could be calculated without any corrections.