Studies have shown that, in patients with threatened premature labor, fenoterol and ritodrine are effective agents for delaying its onset. However, the side-effects of these drugs can be a problem. Among 133 patients studied, the intravenous administration of either agent caused marked tachycardia (but no changes in blood pressure) and a high incidence of palpitations, trembling, and nausea. During oral therapy, subjective symptoms were less frequent. The addition of the calcium antagonist, iproveratril, to the therapeutic regimen did not influence the effectiveness of the 2 agents or the incidence of their adverse side-effects.