The interaction of the antidepressant drug imipramine with insulin in aqueous solution at pH = 3.2 (ionic strength I-c = 6.5 . 10(-3) . mol . dm(-3)) has been investigated by equilibrium dialysis and microcalorimetry. Imipramine binds to insulin to the extent of approximate to(50 to 60) imipramine molecules per insulin molecule in the temperature range T = (283.15 to 308.15) K. The binding data have been used to obtain the Gibbs energy per imipramine molecule bound Delta G(<(nu)over bar>) using the Wyman binding potential approach. The enthalpy of the interaction is small and exothermic following a sigmoidal curve as a function of binding, and Delta G(<(nu)over bar>) is dominated by a large entropy increase on binding which is of similar magnitude to the non-specific hydrophobic binding of several other amphipathic ligands with globular proteins.