In order to determine the influence of calcium on erythropoietin release in response to hypoxia, the effects of the calcium entry blocker verapamil on erythropoietin production were investigated. Hypoxia was induced in male Sprague-Dawley rats by placing them in a hypobaric chamber at 0.42 atmospheres for 6 and 12 hr. Serum levels of erythropoietin were measured by the exhypoxic polycythemic mouse bioassay and a sensitive radioimmunoassay for erythropoietin. Verapamil (5, 10 and 20 mg/kg i.p.) produced significant increases in mean serum erythropoietin levels after 6 and 12 hr of hypoxia, which was significantly higher than those of saline-injected hypoxic control rats. Mean arterial blood pressure values in rats injected with 5 and 10 mg/kg of verapamil were not significantly different from the preinjection controls when measured at ambient pressure over a 12-hr period. A dosage of 20 mg/kg of verapamil i.p. in rats produced a significant decrease in mean arterial pressure between 5 and 30 min after injection but was not significantly different from the vehicle controls between 30 min and 12 hr postinjection. In conclusion, the calcium entry blocker verapamil enhanced erythropoietin production in response to hypoxia. Thus, it is postulated that calcium plays a significant role in erythropoietin production and/or release.