Renal tubule cell hyperplasia and hypertrophy have been regarded as antecedents of tubulointerstitial fibrosis. Dipyridamole and pentoxifylline inhibit rat glomerular mesangial cell growth. The purpose of this study was to examine the effects of pentoxifylline and other vasodilators on opossum kidney proximal tubular (OKP) cell growth. A modified 3-(4,5-dimethylthiazil-2-yl)-2,5-diphenyltetrazolium bromide assay was used to evaluate cell proliferation in OKP cells. Pentoxifylline caused a dose-dependent inhibition of serum-stimulated proliferation of OKP cells: inhibition was 17%, 25%, and 37% at 0.03 mg/mL, 0.1 mg/mL, and 0.3 mg/mL, respectively. Dipyridamole 1.7 mg/L and 6 mg/L resulted in 34% and 46% inhibition, respectively. Verapamil 10(-6) M, 10(-5) M, and 10(-4) M resulted in 34%, 46%, and 67% inhibition, respectively. These inhibitory effects may contribute to renal protection or retardation of renal disease progression.