There are well established bidirectional functional interactions between the central nervous system (CNS) and the immune system. Because of these Links, it is plausible that antidepressant treatment can affect immune function. We evaluated the effect of acute treatment with a tricyclic antidepressant, desipramine, on some immune parameters in the rat. Desipramine treatment (10 mg kg(-1) i.p.) produced a transient rise in total white blood cell count and a reduction in the percentage of lymphocytes, accompanied by an increase in the percentage of neutrophils in the peripheral blood. Acute desipramine treatment also produced a longer lasting increase in leucocyte adhesiveness and aggregation. The changes observed following acute administration of desipramine in the rat are in agreement with a large body of evidence suggesting that the CNS can affect the immune system. This study also demonstrates that in addition to the immunological abnormalities associated with depression, this tricyclic antidepressant causes significant changes in cellular immunity.