Freshly isolated rat hepatocytes show many changes in metabolic activities as a function of cell density in the incubation flask. Fatty acid synthesis, cholesterol synthesis, general protein synthesis, and rates of accumulation of pyruvate, lactate, citrate, acetyl-CoA, acetoacetate and beta-hydroxybutyrate decrease as the cell density increases between 1 mg/ml and 60 mg/ml. Glucose release only decreases between 1-5 mg/ml and the concentration of ATP does not vary at any density. There is a small increase in the lactate/pyruvate ratio and a dramatic decrease in the beta-hydroxybutyrate/acetoacetate ratio with increasing cell concentration. When cells at 8 or 28 mg/ml were incubated with added lactate and pyruvate, or alanine, a two fold increase in fatty acid synthesis and 50% decrease in cholesterol synthesis were observed as compared to rates with endogenous substrate. With added glucose the synthetic rates were similar to those obtained with endogenous substrate. However, regardless of the type of substrate used, the less dense cells gave rates up to three times greater than that of the more dense cells. When conditioned medium isolated after incubation of cells at high density was added to the less dense cells, a decrease in the rates of fatty acid synthesis and cholesterol synthesis was observed in the less dense cells; however, when medium from the less dense cells after incubation for the same period was added to the more dense cells, there was no significant change in fatty acid or cholesterol synthesis. These results suggest that a factor may be released into the medium of incubating hepatocytes that progressively inhibits certain metabolic processes as the cell density increases.