The studies in this report were carried out to investigate the effects of delta 9-tetrahydrocannabinol (delta 9-THC) on cardiac membrane adenylate cyclase activity and to determine the role of changes in membrane lipid order in these effects. delta 9-THC and its psychoactive metabolite, 11-OH-delta 9-THC, increased isoproterenol (ISO) stimulation of adenylate cyclase in rat cardiac ventricular membranes. Cannabidiol, cannabinol and (+)-delta 9-THC were all without effect, indicating that this effect of delta 9-THC is stereoselective and specific for cannabinoids with psychoactive potency. delta 9-THC also increased glucagon stimulation of adenylate cyclase. The enhancement of both ISO and glucagon-stimulated adenylate cyclase was due to an increase in the Vmax of these agonists with no significant change in Kact. delta 9-THC did not affect basal adenylate cyclase activity or the activation of the enzyme by forskolin, guanine nucleotides or fluoride ion. Those cannabinoids which increased ISO-stimulated adenylate cyclase activity also decreased the break temperature of the Arrhenius plot; evidence that the effects of delta 9-THC involve changes in membrane phospholipid order. The effects of the cannabinoids on cardiac membrane phospholipid order were investigated directly using diphenylhexatriene fluorescence polarization. delta 9-THC and 11-OH-delta 9-THC alone decreased the break temperature of the diphenylhexatriene temperature profile, i.e., decreased the temperature of the lipid phase separation. This effect of delta 9-THC was stereoselective.(ABSTRACT TRUNCATED AT 250 WORDS)