1. The spleen contains approximately one-third of all the body's platelets. These platelets are relatively larger and haemostatically more active than platelets in the systemic circulation and can be released into the systemic circulation by stimulation of alpha-adrenoceptors or inhibition of beta-adrenoceptors. In the present study, we evaluated the effects of selective (bisoprol) and non-selective (carvedilol) beta-blockers agents on mean platelet volume (MPV) and spleen size in hypertensive patients at rest and after exercise. 2. Blood pressure, heart rate, platelet count, MPV and spleen volume were measured in 18 newly diagnosed hypertensive patients, as well as in nine healthy control subjects, subjected to treadmill exercise test at their first visit and, for the hypertensive group, after 15 and 30 days of treatment with the selective beta(1)-adrenoceptor antagonist bisoprolol 5 mg/day (n = 9) or the non-selective alpha(1)-, beta(1)- and beta(2)-adrenoceptor antagonist carvedilol 25 mg/day (n = 9). 3. Increases in resting MPV values with concomitant decreases in spleen volume were found after 15 and 30 days treatment with either bisoprolol or carvedilol. The pronounced decrease in splenic volume after exercise and the increased MPV and platelet counts seen at first visit were halved after 15 and 30 days of treatment with either drug. 4. We conclude that in hypertensive patients treated with either selective or non-selective beta-blockers, the spleen contracts and MPV increases, which may increase the risk of atherothrombosis.