To investigate the relationships between cardiovascular variables (SBP, DBP, and HR) and circulating natural killer (NK) cell numbers, 70 male volunteers were subjected to a rest condition (N = 30) or a stressful laboratory task (N = 40). At baseline, no significant relationships could be demonstrated between the number of NK cells and the cardiovascular variables. Analysis of covariance showed that the stressor induced increases in the number of NK cells, SBP, DBP, and HR. Changes in NK cell numbers were highly correlated to changes in cardiovascular variables in both the task and the no-task group. These results indicate that there is no relationship between the number of circulating NK cells and cardiovascular levels per se, but that changes in these variables, either stress-induced or under rest conditions, are regulated by a common mechanism.