Treatment of platelets with a prostacyclin analogue, iloprost, decreased the cholera-toxin-induced ADP-ribosylation of membrane-bound Gs alpha (alpha-subunit of G-protein that stimulates adenylate cyclase; 42 kDa protein) and a cytosolic substrate (44 kDa protein) [Molina y Vedia, Reep & Lapetina (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 5899-5902]. This decrease is apparently not correlated with a significant change in the quantity of membrane Gs alpha, as detected by two Gs alpha-specific antisera. This finding contrasts with the suggestion in a previous report [Edwards, MacDermot & Wilkins (1987) Br. J. Pharmacol. 90, 501-510], indicating that iloprost caused a loss of Gs alpha from the membrane. Our evidence points to a modification in the ability of the 42 kDa protein to be ADP-ribosylated by cholera toxin. This modification of Gs alpha might be related to its ADP-ribosylation by endogenous ADP-ribosyltransferase activity. Here we present evidence showing that Gs alpha was ADP-ribosylated in platelets that had been electropermeabilized and incubated with [alpha-32P]NAD+. This endogenous ADP-ribosylation of Gs alpha is inhibited by nicotinamide and stimulated by iloprost.