Tissue monoacylglycerols (MG), diacylglycerols (DG), free fatty acids (FFA), and cyclic AMP (cAMP) and release of FFA and glycerol have been studied in vitro in subcutaneous adipose tissue of 6 obese and 7 normal-weight subjects. The tissue was incubated without or with 6 X 10(-5) mol/l of isoprenaline (ISNA). The DG level and the fat cell volume were strongly interrelated (r=+0.95, p less than 0.001). The concentration of DG was increased (p less than 0.05) in obesity. The changes in DG and MG were significantly interrelated (r=+0.65, p less than 0.05) during basal incubation. ISNA increased the DG concentration in a way that was correlated (r=+0.81, p less than 0.001) with the ISNA-induced glycerol release. This indicates that 1) the basal metabolic activities of MG and DG lipase are similar and 2) DG lipase is an important rate limiting factor in lipolysis. Without ISNA, tissue FFA and the release of FFA and glycerol were significantly increased in the obese patients. As a mean, MG and DG did not accumulate in the basal state in the two patient groups. The findings indicate that basal lipolysis was increased in obesity. This was probably due to increased basal metabolic activity of triacylglycerol lipase, since the basal cAMP levels were similar in the two patient groups. In the presence of ISNA, the production of FFA and the glycerol release were similar in both patient groups, as was the increase in tissue DG. Also the ISNA-induced maximal level of cAMP was similar in the two groups. With ISNA, a small increment of MG was observed in adipose tissue of the normal-weight subjects. Taking all metabolites into account, the rate of lipolysis as well as the activation of triacylglycerol lipase via cAMP in the presence of ISNA appeared to be unaltered in obesity. Separate experiments with 1-14C-glycerol provided further evidence for the existence of a MG pathway for the esterification of FFA.