The effect of forskolin on platelet-activating factor (PAF) receptor was investigated. Rabbit platelets treated with forskolin showed approximately a 9-fold increase in cAMP levels over the control. After treatment of platelets with forskolin prior to PAF binding, a 30-40% (P < 0.005) decrease in PAF binding was observed. The decrease in PAF binding caused by forskolin was concomitant with a decrease in the physiological responses of platelets induced by PAF. However, this forskolin-induced decrease in PAF binding was not a consequence of cAMP formation as the addition of a cAMP analog could not mimic the action of forskolin. Additionally, the inactive analog of forskolin, dideoxyforskolin, which does not activate adenylyl cyclase, also reduced PAF binding to its receptor. Reduction of PAF binding by forskolin and dideoxyforskolin was also observed with isolated platelet membranes. To understand the mechanism of forskolin induced changes in PAF binding, the involvement of a G-protein in this process was investigated. Cells treated with GTP gamma S showed approximately a 25% reduction in PAF binding. Addition of forskolin to the GTP gamma S treated cells resulted in a further reduction in PAF binding, suggesting the action of forskolin was independent of G-protein activation. The data suggests that the action of forskolin was independent of adenylyl cyclase or G-protein involvement. It is speculated that the action of forskolin on PAF binding was due to a direct effect of this molecule and its analog on the PAF receptor itself or to components of the post-receptor signalling for PAF.