The effect of mental arithmetic on beta 2-adrenoceptors on lymphocytes, as measured by (-)125I-iodocyanopindolol binding, was studied in normotensive and essential hypertensive females. To determine the role of adrenaline, mental arithmetic was also carried out in bilaterally adrenalectomized females. There were no significant differences in the blood pressure and heart rate responses to mental arithmetic among the three groups. Baseline values or changes in adrenaline levels during mental arithmetic were similar in normotensives and hypertensives, whereas levels of adrenaline remained undetectable throughout the test in adrenalectomized females. Baseline values of beta 2-adrenoceptors did not differ among the three groups. In normotensive and hypertensive females the increase in beta 2-adrenoceptor density was, respectively, 27 and 20%. Baseline values of beta 2-adrenoceptors were not related to the percentage changes in the receptor densities. In adrenalectomized females, in contrast to normotensives and hypertensives, there was no change in receptor density. These findings suggest that adrenaline plays a role in the short-term upregulation of beta 2-adrenoceptors after mental arithmetic.