The effects of single and repeatedly administered amitriptyline (10 mg/kg ip) on the ability of the splenocytes of the restraint-stressed (for 2.5 h) Sprague-Dawley rats to induce the Graft versus Host (GvH) reaction in vivo and to respond to the concanavalin A (Con A) stimulation in vitro were studied. In acute experiments single immobilization, amitriptyline administration, or single amitriptyline treatment of the previously stressed rats did not change the reactivity of splenocytes to both mitogen and alloantigens. On the other hand, in the "chronic" experiment, when rats were stressed 2.5 h daily for five days with or without concurrent amitriptyline treatment, a statistically significant decrease in the reactivity of splenocytes to Con A and increase in the reactivity to alloantigens were visible only in the stressed, but not in stressed, amitriptyline treated animals.