Prostaglandin A1 inhibits the energy-dependent export of cyclic AMP from avian red cells. When exposed to PGA1, avian red cells rapidly accumulate the prostaglandin and convert it to a polar metabolite. Inhibition of cyclic AMP efflux correlates with the intracellular content of this metabolite. Data in the literature and our chemical and chromatographic analyses indicate that the prostaglandin metabolite is a glutathione adduct of PGA1. Thus, we hypothesize that a GSH adduct of PGA1 acts intracellularly to elevate cyclic AMP by blocking its extrusion.