In August rats, a strain highly susceptible to emotional stress, 24 h immobilization stress (IS) decreased the rate and amplitude of platelet aggregation induced by collagen (2–4 m̈g/ml) or ADP (0.25–5.0 m̈M) in platelet‐rich plasma, but had no effect on ATP release reaction and calculated peripheral blood platelet counts. Platelet aggregability was unchanged in groups deprived of food and water for 24 h or subjected to a 6 h IS. Only after 24 h IS were the effects of collagen and low concentrations of ADP potentiated by preincubation with epinephrine (1 m̈M). The IS‐induced decrease of platelet aggregation significantly correlated with the thymus involution, and both effects were most pronounced in rats displaying low scores of activity in the hole–board apparatus. The data indicate that behavioural variables should be taken into account in the analysis of stress‐induced changes in platelet functions, and possibly of any other stress response.