Isolated rabbit atria were employed to examine the rate of onset of inotropism of two glycosides (strophanthin and ouabain) and aglycones (acetylstrophanthidin and ouabagenin) under a variety of conditions of temperature and beat interval. The rate of onset of glycoside-induced inotropism was found to be highly and directly dependent on temperature between 27 and 37 degrees C at a beat interval of 10 seconds. At a beat interval of 1 second the rate of onset of glycoside effect was faster than at a beat interval of 10 seconds and was not dependent on temperature. At 32 degrees C the rate of onset of glycoside-induced inotropism was inversely related to beat interval (the faster the stimulation rate, the faster the onset). The rate of onset of aglycone-induced inotropism was not dependent on temperature at a beat interval of 1 or 10 seconds. At 32 degrees C the rate of onset of aglycone effect was not dependent on beat interval except at beat intervals of less than 1 second. Depending on experimental conditions, increased temperature and/or myocardial activity increases the rate-limiting step for the onset of effect of cardiac glycosides. Rapid myocardial activity increased, but neither moderate myocardial activity nor increased temperature increase the rate-limiting step for the rate of onset of effect of aglycones under our experimental conditions. The results are interpreted by postulating that the digitalis inotropic receptor is located in a membrane-limited compartment to which aglycones may gain access via passive diffusion but to which glycosides gain access mainly via a carrier mechanism.