The growth and development of total body water, fat, ash and protein were examined in lean and obese Zucker rats (fa/fa) from 13 to 31 days of age. The obese rats began depositing significantly more fat compared to the lean controls by day 13. At this time the obese had 3.8% more carcass fat than the lean, with this difference increasing to almost 13% by day 31. Per cent ash and protein were significantly lower in the obese rats throughout the 18 day experimental study. However, when the absolute grams of these two parameters were expressed on a fat-free weight basis, no differences were recorded between the genotypes. There were also no differences when the slopes of the linear log-log growth equations for fat-free weight versus ash and protein were compared. These data do not lend support to the contentions that skeletal stunting and decreased protein deposition are present early in the development of the obese rat. A technique to identify obese rats at an early age without the necessity of killing them, by determining total body water with injected tritiated water was also examined. The method correctly identified obese and lean rats at 2 weeks of age. Further refinement of this technique is necessary before it may be used to precisely predict the genotype of a rat.