Secretin and VIP stimulate cardiac adenylyl cyclase activity and exert a positive inotropic action in several mammalian species. This study examined positive inotropic activity and cAMP levels in rat papillary muscle. Isoproterenol and secretin increased contractions by 150 ± 31% and 129 ± 27%, respectively. VIP increased contraction by 30 ± 21% only at 10 μM. Isoproterenol significantly increased cAMP levels by 82%, whereas increases by secretin (58%) and VIP (56%) were not significant. These results are consistent with reports that secretin and VIP stimulate cardiac adenylyl cyclase in the rat, but suggest that cAMP tissue levels cannot totally explain the positive inotropic responses to secretin and VIP.