The dissociation of [3H]imipramine from human platelet membrane recognition sites was studied. Simple exponential dissociation kinetics were observed indicating an apparent homogeneous class of sites which dissociate by a first order rate process. Inclusion of serotonin in the medium dramatically decreased the dissociation rate. At 0 degree C half-times of dissociation were 61.7 and 170 min in the absence and presence of serotonin, respectively. Other biogenic amines were ineffective in altering the dissociation kinetics. It is suggested that serotonin controls [3H]imipramine dissociation from an allosterically coupled site residing on a larger macromolecular complex.