Both verapamil and nifedipine were found to inhibit various types of activation of the isolated rat uterus, as for example: tonic and phasic components of contraction produced by KC1 and by oxytocin, phasic components of contraction produced by electrical stimulation and phasic spontaneous rhythmic activity. Presumably, verapamil and nifedipine affect a common pathway in calcium metabolism in all three types of muscle activation. Both verapamil and nifedipine inhibited more the contractile responses to high concentrations of KC1 than the responses to oxytocin, indicating the possibility of KCI and oxytocin acting through different calcium channels. Nifedipine inhibited more the tonic than phasic components of contractions produced by both KCI and oxytocin. It is assumed that two different calcium channels might be implicated in two components of contractions produced by KCI and oxytocin. Phasic components of contraction produced by KCI and by electrical stimulation were almost completely restored by increasing the concentration of calcium in the medium, whereas tonic components were restored only to about 50%.