The micellar properties of the tricyclic antidepressant drug clomipramine hydrochloride in aqueous solution have been examined using ultrasound velocity, density, and light-scattering techniques and compared with literature values for the structurally related drug, imipramine hydrochloride. Although the two drugs show minimum CMCs at similar temperatures (26 and 21 degrees C for clomipramine and imipramine, respectively), the variation of the CMC of clomipramine with temperature change was less pronounced than that of imipramine. The changes of apparent molar volumes and adiabatic compressibility accompanying micelle formation of clomipramine were similar to those of imipramine and appreciably smaller than those for the micellization of typical surfactants; the lower values of adiabatic compressibility for the micellization of the drugs suggest more tightly packed aggregates. Addition of electrolyte (0.025-0.100 mol dm(-3) NaCl) caused a significant increase of aggregation number and a decrease of the degree of ionization of clomipramine, but had little effect on the micellar properties of imipramine.