Genetically hypertensive (LH), normotensive (LN), and low blood pressure (LL) rats of the Lyon strains have been simultaneously selected according to their systolic blood pressure (SBP). SBP and immunological parameters were investigated in 5 week-old rats. SBP reached 99 +/- 2 mmHg in LL, 108 +/- 2 mmHg in LN and 122 +/- 4 mmHg in LH. White blood cell and lymphocyte counts, thymus and spleen histology, percentages of T cells and subsets of T cells in these organs were not modified by genetic hypertension. The blastogenic responses to mitogens (Con A and PWM) of the LH rat thymocytes were similar to those observed in LN or LL rats. By contrast, the proliferative response of splenocytes to PWM was positively correlated (r = 0.93) with SBP level. In additional experiments we demonstrated that, besides the immune defect induced by neonatal thymectomy, the removal of the thymus at birth prevented the spontaneous increase of SBP in LH strains (102 +/- 2 mmHg instead of 122 +/- 4 mmHg), while only a slight decrease or no alteration was seen in LN and LL thymectomized rats. This antihypertensive effect tended to maintain SBP at a common level in the 3 strains. These results suggest that the thymus may be involved in the development of genetic hypertension.