Various purine and pyrimidine ribosides, with the exception of adenosine, have been found to exhibit a positive inotropic effect on an isolated strip of frog ventricle. In a preparation responding maximally to one compound, the addition of a second compound led to a further increase in tension. This relationship was found to apply among the members of a given group (purine or pyrimidine) paired in various combinations, as well as in experiments pairing purine against pyrimidine riboside. Adenosine showed brief stimulation followed by depression, pure depression or no effect.