Isolated tissues from rat and guinea pig hearts were employed to investigate the relationship between the positive inotropic effects of isoproterenol and glucagon and the shortening of time to peak tension (TPT). The involvement of cAMP in the inotropic responses to histamine and isoproterenol and corresponding changes in TPT was also studied. On isoproterenol challenge, a correlation was found between the percent increase in tension and reduction in TPT. Histamine produced a positive inotropic response in both guinea pig left atria and papillary muscles; however, TPT was shortened only in the cAMP-associated response of the papillary muscle. In guinea pig left atria, isoproterenol in the absence and presence of D-600, RO-20-1724, or propranolol increased tissue levels of cAMP and decreased TPT such that a significant correlation was found between these two parameters. The significance of relationships among positive inotropic responses, tissue levels of cAMP, and TPT is discussed.