To compare the effects of enalapril and captopril on blood pressure, serum creatinine (S-Cr), and potassium (S-K) levels, patients with malignant hypertension treated with either 5-10 mg of enalapril (eight cases) or 75-400 mg of captopril (eight cases) were investigated retrospectively. After 2 weeks of treatment, the average blood pressure fell from 214/138 to 132/89 mmHg on enalapril and from 240/145 to 147/95 mmHg on captopril. The percent change in mean blood pressure during the 2 weeks of treatment with enalapril (-35.6 +/- 4.0 SE%) was similar to that with captopril (-35.8 +/- 2.8%). S-Cr did not change in both groups, while S-K increased significantly from 3.9 +/- 0.2 to 5.2 +/- 0.2 mEq/l on enalapril and from 3.6 +/- 0.2 to 4.2 +/- 0.1 mEq/l on captopril. S-K at the second week was significantly higher in the enalapril than in the captopril group. The maximum S-Cr concentration during the treatment was correlated with the corresponding S-K concentration similarly in both groups. These results indicate that both enalapril and captopril increase S-K without deterioration of renal function in patients with malignant hypertension and that the way these drugs are used in clinical practice may be more likely to result in elevated S-K with enalapril.