The apparent molal volumes and adiabatic compressibilities of aqueous solutions of the amphiphilic antidepressants amitriptyline, nortriptyline, and desipramine have been determined from density and ultrasound velocity measurements. Critical concentrations were obtained from both techniques. Positive deviations of the apparent molal volume of nortriptyline from the Debye-Huckel limiting law in dilute solution indicate premicellar association. The changes of molal volume accompanying aggregate formation suggest a tightly packed aggregate. Isentropic apparent molal adiabatic compressibilities were calculated by combining the ultrasound velocity and density data. Changes of this quantity were similar to those of typical surfactants, indicating a decrease of hydrophobic hydration in the association of the monomers of these drugs.