A study of associations between lactate dehydrogenase and cytochrome oxidase isoenzymes and total cytochrome oxidase activity, on the one hand, and the type of arterial hypertension, the magnitude of blood pressure and its response to treatment, on the other, was carried out in 143 patients with varying stages of essential hypertension and in renal hypertension. The examined parameters were found to be valuable indicators of adequate hypotensive effect and an optimum BP fall. A 20-24% BP drop below the baseline is optimal for patients with renal hypertension. A 30% BP drop is acceptable for patients with stable essential hypertension. Attaining normal BP level is acceptable and desirable for cases of labile essential hypertension.