The positive effect of phenylephrine (PE) on stroke volume (SV) was 3 to 5 times weaker in the rats exposed to hypokinesia for 30 days as compared to the controls. After obsidan blockade of beta-adrenoreceptors SV increased in both groups and the intergroup differences in the PE effect remained significant but less pronounced. This can be attributed to a greater effectiveness of PE after obsidan administration during hypokinesia. Correlation analysis showed that the weak effects of PE on SV were potentiated by obsidan (potentiation was the stronger the weaker the effects) while the distinct effects were on the contrary inhibited. This demonstrates the synergy--antagonism relationships in the PE and obsidan interaction and seems to indicate that the common site of their action is alpha 1-adrenoreceptors. After phentolamine injections the PE effect on SV was not found. In this situation SV decreased; the SV decrease as well as its increase under the action of obsidan was less significant in the hypokinetic than in the control rats. The above investigation suggests that the activity of alpha 1-adrenoreceptors involved in the actualization of positive effects of agonists on SV is considerably lower during hypokinesia.