The effect of the ob=ob mutation on microsomal overt and latent acyl CoA:diacylglycerol acyl transferase activities (DGATs I and II) in liver microsomes of mice was investigated. Overt and latent hepatic DGAT activities for 16-week-old lean Ob=? mice were 0.42 AE 0.14 and 1.57 AE 0.04 mmol=mg=min, respectively. For ob=ob mice DGAT I and II activities were 1.17 AE 0.28 and 3.09 AE 0.78 mmol=mg=min, respectively. The hepatic triacylglycerol (TAG) concentration of 16-week-old mice was increased three-fold in ob=ob mice relative to those in lean controls. The data suggest that the increased rate of hepatic TAG secretion and intracellular accumulation in ob=ob mice is accompanied by parallel increases in the activities of both DGATs.