The uptake and release of [3H]desipramine, an antidepressant drug, was investigated in cultures of human fibroblasts during single and repeated doses added to the culture medium. The resulting kinetics and the pH dependence suggest that desipramine is a lysosomotropic drug which is taken up into the cells by virtue of its amphiphilic character and concentrated in the acidic contents of the lysosomes by ion trapping. Similar to the lysosomotropic agent, chloroquine, desipramine interferes with the lysosomal degradation of endogenous sulfated mucopolysaccharides in a dose-dependent manner. In contrast to short time exposure, long-term treatment with desipramine produces intracellular granulation, possibly by interfering with the recycling of plasma membrane vesicles. The appearance of granular inclusions and the accumulation of intracellular desipramine which is difficult to exchange indicate a gradual formation of complexes with membrane phospholipids resulting in a lysosomal storage of desipramine.