Felodipine is a new dihydropyridine calcium antagonist, and in hypertension it is a much more effective "third-line" drug than hydralazine. Nifedipine, on the other hand, is the established dihydropyridine calcium antagonist that has been increasingly used to treat hypertension. Information is now needed on the relative merits and demerits of these two drugs. This study appraised, therefore, the therapeutic utility of twelve months' treatment with nifedipine 20-60 mg twice daily in 55 patients with previous drug-resistant hypertension who had been successfully treated for the previous year with felodipine 5-20 mg twice daily, each calcium antagonist being used in combination with atenolol 100 mg daily with or without chlorthalidone 25 mg daily. Initially, nifedipine maintained comparable blood pressure control to that which had been achieved by felodipine, although in the longer term (over eight months) nifedipine proved less effective than felodipine had (p less than 0.02) and more patients became uncontrolled (supine diastolic blood pressure, Phase V, greater than or equal to 90 mmHg) on the maximum tolerated dose of the calcium antagonist (chi 2 = 4.13, p less than 0.05 greater than 0.025). The former degree of blood pressure control was, however, reestablished by increasing the dose of nifedipine or reintroducing the diuretic as necessary, and this control was maintained over the next four months. Minor side effects were less common on nifedipine than they had been during the preceding felodipine treatment phase. Felodipine thus has more pronounced and sustained antihypertensive effects than nifedipine, though its side effect burden may appear to be greater.(ABSTRACT TRUNCATED AT 250 WORDS)