The formation of triacylglycerols containing fenoprofen was studied in rat isolated adipocytes and hepatocytes incubated with [3H]glycerol and R or S fenoprofen. In both hepatocytes and adipocytes there was a high-affinity enzymatic process for the synthesis of triacylglycerol containing fenoprofen which was stereospecific for the R enantiomer. The apparent Km values for R fenoprofen were 1.0 μM in adipocytes and 2.8 μM in hepatocytes. These results are consistent with the proposed stereospecific formation of R-2-arylpropionyl-CoA thioesters resulting in the stereospecific formation of R-tri-acylglycerol at clinically relevant unbound fenoprofen concentrations. In isolated hepatocytes, but not adipocytes, a second low-affinity enzymatic process for the synthesis of triacylglycerol containing fenoprofen was also observed. However, this process (Km = 3780 μM) occurred at concentrations much higher than those found in man with usual doses.