Natural killer (NK) cell activity, cortisol and catecholamine levels, and physiological reactivity were examined in 15 healthy women who volunteered to take a cognitive stress test (the Stroop test). Relationships were explored among the physiological and selected psychosocial variables. Urine and blood samples were taken to examine catecholamine and cortisol concentrations and NK cell activity immediately before, immediately after, and hourly for 6 hours after the Stroop test. During the Stroop test, heart rate, skin conductance, peripheral skin temperature, and blood pressure were measured. Although skin conductance, heart rate, and blood pressure increased in response to the Stroop test, neuroendocrine values did not. Cortisol secretion decreased after the Stroop test and appeared to follow the normal circadian rhythm. NK cell activity was variable among individual participants but tended to increase over time.