Eritadenine was investigated for its effects on adenylate cyclase activity of rat fat cell plasma membranes on cyclic AMP accumulation and lipolysis in isolated rat fat cells. In rat fat cell plasma membranes, a 50% inhibition of noradrenaline (100 mumol/l) stimulated adenylate cyclase was obtained at 11.6 mumol/l eritadenine or 9.0 mumol/l adenosine. NaF (3 mmol/l) stimulated adenylate cyclase was inhibited at concentrations of eritadenine (IC25 5.8 mumol/l) lower than those of adenosine (IC25 51.9 mumol/l). Eritadenine ethyl ester (100 mumol/l) was almost ineffective on adenylate cyclase. The inhibitory effect of eritadenine was resistant to adenosine deaminase. Isolated rat fat cells eritadenine (100 mumol/l) was completely ineffective to block noradrenaline-stimulated cyclic AMP accumulation or lipolysis stimulated by theophylline or adenosine deaminase. It is suggested that eritadenine is an effector of the adenosine P site of fat cell plasma membranes.