In this study platelet aggregation was measured by the screen filtration pressure method in 2 groups of hypertensives separated according to age. They were perfectly matched for levels of various "risk factors", with normotensive "controls". No significant difference in platelet aggregate filtration pressure (PAFP) was found between hypertensives and normotensives. Contrary to normotensive subjects, there was no significant increase in the PAFP values with age in the hypertensives. This might be due to the significantly higher blood urea values in the hypertensives as compared to the normotensives. An inverse relationship between blood urea concentration and PAFP values was in fact confirmed by in vivo and in vitro studies.