Renal interstitial scarring is an important feature of heroin-associated nephropathy. We studied the effect of morphine, an active metabolite of heroin, on cultured rat renal medullary interstitial cell (RMIC) proliferation and matrix accumulation. Morphine (10<sup>-12</sup> M) enhanced (p < 0.001) the proliferation of RMIC (control, 15.0 ± 0.5 vs. morphine, 20.4 ± 1.1 × 10<sup>4</sup> cells/ml). This effect of morphine was dose and time dependent. [<sup>3</sup>H]thymidine and bromodeoxyuridine incorporation studies confirmed the mitogenic effect of morphine on RMIC. Morphine also enhanced mRNA expression for c-jun and c-myc on RMIC. However, nalbuphine, a non-addicting alkaloid did not modulate the proliferation of RMIC. Morphine enhanced the accumulation of collagen type I in a dose-dependent manner and also increased (p < 0.001) the accumulation of collagen type III at a high concentration (control, 1,291 ± 55.8 vs. morphine, 10<sup>-4</sup> M, 2,697.6 ± 257.8 ng/μg protein). Morphine did not modulate the accumulation of laminin or fibronectin. Neutralizing antibody to IL·6 inhibited the effect of morphine on RMIC. H7, a protein kinase C inhibitor, also attenuated the morphine-induced RMIC proliferation. The present study provides a basis for a hypothesis that morphine may be playing a role in the development of renal interstitial pathology in patients with heroin addiction.