Metabolic adaptations to cyclic patterns of food intake were studied in genetically lean and obese Zucker rats. Twenty-four lean and 24 obese rats were exposed to 12 hours of light and 12 hours of dark and allowed food ad libitum. Both groups of rats ate more during the dark period of the cycle. The obese consumed nearly twice as much food as the lean during the light period of the cycle. At 4-hour intervals, rats were killed and liver and epididymal fat pads were removed for metabolic studies. Adipose tissue from lean rats demonstrated marked changes in rates of lipogenesis during the 24-hour cycle whereas adipose tissue from obese rats maintained a relatively steady rate of lipogenesis. Glucose incorporation into the glycerol moiety of triacylglycerol was nearly 3-fold higher in adipose tissue from obese rats. Liver lipogenesis in lean and obese rats followed their food intake pattern. Liver lipogenic rate (expressed per organ) was 3- to 5-fold higher in obese than lean rats during most of the 24-hour cycle. These data support the concept that the excessive fatty acids produced in the liver of obese rats are being esterified by adipose cells. Lipolytic response to glucagon was found in adipose tissue from obese rats during the dark and light periods, but only during the dark period for lean rats. These data suggest, in comparison to lean rats, that obese rats do not enter a relative catabolic state during a 24-hour cycle. A constant anabolic state in the genetically prone individual may lead to excessive lipid deposition and obesity.