In rat fat cells incubated for 15 min at 37 degrees and pH 8.5, glycerol release was highly stimulated both by norepinephrine and by theophylline. Prostacyclin (PGI2) (10(-8)-10(-7)M) did not alter the basal rate of glycerol release but potentiated the lipolytic effect of 2 X 10(-6)M norepinephrine. The rate of norepinephrine-induced glycerol release was increased by PGI2 during 10 min of incubation and then maintained for the next 5 min. Lipolysis induced by concentrations of norepinephrine which produced maximal effects was not altered by PGI2. PGI2 (10(-7)-10(-6)M) also potentiated the effect of 5 X 10(-4)M theophylline on glycerol release, but antagonized the stimulation induced by a maximally effective concentration of the methylxanthine (2 X 10(-3)M). Incubation of the cells with norepinephrine in the presence of 2 X 10(-4) or 5 X 10(-4)M theophylline caused a loss of the potentiating effect of PGI2 on norepinephrine-induced lipolysis. In the presence of 10(-3)M theophylline, the lipolytic action of norepinephrine was inhibited by PGI2. In fat cells incubated with adenosine deaminase (0.5 U/ml), 2.5 X 10(-7)M PGI2 did not alter the response to 5 X 10(-4)M theophylline and inhibited the effect of norepinephrine both in the absence and in the presence of theophylline. The present results show that, under appropriate experimental conditions, PGI2 may act as a lipolytic agent in isolated fat cells and that some kind of interaction exists between stimulation of methylxanthine-sensitive adenosine receptors and stimulation of PGI2 receptors.