The effect of terbutaline sulfate on calcium transport across muscle strips isolated from rat diaphragms, and its effect on this process when used in combination with aminophylline was investigated using labelled calcium in a two-compartment system. The results show that terbutaline sulfate causes a dose-dependent increase in intracellular calcium concentration which reached saturation at a terbutaline sulfate concentration of 2 ng/ml. The inclusion of both aminophylline and terbutaline sulfate at a concentration range of 6.25-50 micrograms/ml and 0.5-4 ng/ml respectively in the incubation medium, did not show a synergistic effect on intracellular calcium accumulation. Although verapamil, a calcium channel blocker, inhibited significantly (P less than 0.01) calcium accumulation in the muscle strips of the diaphragm, it did not inhibit completely the increase in intracellular calcium concentration noticed in the presence of aminophylline or terbutaline sulfate. This would suggest that aminophylline and terbutaline sulfate have an effect on the calcium channels in the muscle strips of the diaphragm.