Platelet [³H] paroxetine binding was measured in 73 depressed patients and in 64 healthy volunteers. No differences were found in B_max or K_d either overall, or when the 61 depressed subjects who had never received psychotropic drugs were analysed separately. Within the depressed group, no differences in B_max or K_d were found between subgroups divided on the basis of endogenicity, suicidal thoughts or severity of depression. None of the subgroups differed significantly from controls. Forty of the depressed subjects were retested after 6 weeks' treatment with fluoxetine (n=22) or lofepramine (n=18). Treatment was not associated with any change in B_max but a similar and significant increase in K_d was noted following treatment with either antidepressant. Neither pre- nor post-treatment platelet binding parameters appeared to relate to clinical response to treatment.