Our aim was to assess clinically whether there was any benefit in adding a single dose of sublingual nifedipine (a slow calcium channel blocker) to prazosin in the management of the cardiovascular manifestations of envenoming by the Indian red scorpion (Mesobuthus tamulus). A total of 163 patients stung by this species was admitted to hospital at Mahad between January 1991 and October 1993. Cardiovascular abnormalities were hypertension (59), of whom 42 had bradycardia and 17 had tachycardia; pulmonary oedema (14), of whom eight had hypertension and six hypotension; supraventricular tachycardia (eight), of whom three had hypotension and one died. Of the remaining patients, 78 demonstrated severe excruciating local pain at the site of sting but had no systemic involvement. Nineteen patients with hypertension and tachycardia were given a single dose of sublingual nifedipine plus prazosin on admission, then prazosin alone repeated 6 hourly. Five patients with massive life-threatening pulmonary oedema recovered after being given intravenous sodium nitroprusside. Prazosin alone helped to alleviate cardiovascular manifestations in the remaining 52 victims. One patient was admitted in a deep coma, 12 hr after the sting, and died. Eight victims whose blood pressure had been controlled in hospital by nifedipine plus prazosin developed acute pulmonary oedema necessitating additional doses of prazosin for recovery. Fifty-two victims treated with prazosin alone did not develop pulmonary oedema and the drug appeared to hasten the recovery. In the presence of high blood pressure, tachycardia, a murmur and impending myocardial failure, nifedipine appeared to contribute to cardiopulmonary instability and to augment myocardial oxygen consumption. In this situation calcium channel blockers should probably be avoided.