Effects of 3 oximes, diacetylmonoxime (DAM), pyridine-2-aldoxime (PAM) and pyridine-2-aldoxime methochloride (2-PAM), on the normal electrical and mechanical activities and on the slow response action potentials were examined in the guinea pig ventricular muscles. DAM and long-term exposure to high concentrations of PAM produced decreases in contractile force, action potential duration and slow response action potentials, whereas 2-PAM and low concentrations of PAM tended to increase these parameters. Thus, these 3 oximes did not act uniformly on cardiac muscle. It was speculated that DAM and high concentrations of PAM may act as slow channel inhibitors, whereas 2-PAM and low concentrations of PAM may act as slow channel activators.