The phenolsulfotransferase (PST) is the main sulfoconjugating enzyme of catecholamines. We have described a standardized, sensitive, and reproducible method of measurement of PST activity from human blood platelets. The affinity of PST was, in descending order, towards dopamine (DA) greater than norepinephrine (NE) greater than epinephrine (E) (Km (DA) equals 17.6 microM, Km (NE) equals 66.6 microM, Km (E) equal 330 micro M). The optimum pH was found to be 5.9. Platelet PST measurements in 26 subjects led to the following observations, (i) The individual PST activity remains relatively stable during manipulations such as direct venous puncture, upright posture, physical exercise, and consumption of a copious meal. (ii) The mean platelet PST activity was 67 plus or minus 34 (mean plus or minus SD) U/mg protein in men and 43 plus or minus 19 U/mg protein in women. (iii) The platelet PST activity varies during the menstrual cycle with a marked decrease 7-10 days before menstruation. (iv) Essential hypertensive patients, appearing clinically as a syndrome imitating pheochromocytoma, had absent or low conjugated catecholamines. In one of them, no PST activity could be detected in platelets using catecholamines as substrates whereas a considerable activity was present with p-nitrophenol. The potential usefulness of the platelet PST measurements is discussed.