AbstractPlasma cortisol, growth hormone and prolactin were measured together with lymphocyte subpopulation numbers in nine volunteers in good health during the performance of a laboratory task, and five of these subjects were tested during an otherwise identical control period where the task was not performed. Psychological and autonomic measures indicated that the task was found to be moderately arousing and stressful by the subjects. Although homonal changes were not statistically significant because of the variability of individual responses, consistent patterns in the responses were observed. Plasma cortisol decreased progressively on the task day whereas on the non‐task day cortisol levels rose during the post‐task period. On both days growth hormone levels rose prior to the task and fell progressively afterwards. Prolactin levels fell throughout the procedure on both days. There were significant falls in all lymphocyte subpopulations between the initial and basal sample (p<0.02) and the counts rose again subsequently on both task and non‐task days. It is suggested that the effect of an initial anticipatory reaction confounded the effects of the task on plasma hormone and lymphocyte dynamics. The possible implications of the results of immune function are disc