This study examined the effect of immobilization stress on the expression of muscarinic and beta-adrenergic receptors on thymocytes and lymphocytes obtained from 5-month-old L-E male rats. After 2 h immobilization (acute stress) there was a significant increase in specific binding of [H-3]-DHA to beta-adrenergic receptors on thymocytes and on lymphocytes from the blood but not from the spleen, whereas [H-3]-QNB binding to muscarinic receptors in those cells was not altered in comparison with the undisturbed control. Chronic immobilization stress (5 days, for 2 h) decreased the [H-3]-QNB binding to lymphocytes collected from the spleen and blood but not from thymus; it caused neither a significant change in the H-3-DHA binding to thymocytes nor lymphocytes obtained from the blood and spleen.