In our previous experiments studying the effects of FK506 on renal allografting in the dog, we encountered two major problems. One problem was anorexia and the other problem was vascular changes mainly in the recipient heart. Anorexia was generally dose dependent, but the vascular changes were seen to be more prominent at lower doses rather than at higher immunosuppressive doses. The present study was undertaken to study these two problems. A nonanorexic, vascular change-related, nonimmunosuppressive dose of FK506 was combined with a low dose of cyclosporine or prednisolone in beagle dogs after renal allografting.Treatment with either FK506 alone at a dose of 0.32 mg/kg or cyclosporine alone at 2.5 mg/kg was not effective in prolonging renal recipient survival. The recipient dogs died of rejection, and a variety of vascular changes were observed in the hearts of both groups. Combined treatment with FK506 and cyclosporine at these same doses resulted in statitsitcally significant prolongation of the survival time of the renal recipient (P<0.01), and histologic studies showed that the frequency and severity of the vascular changes were suppressed in the recipient receiving the combined treatment. The combination of FK506 and prednisolone at 0.5 mg/kg was not effective in prolonging survival. Furthermore, the extent of vascular changes was similar to those found in recipients receiving FK506 alone.The data suggest that combined treatment with low doses of both FK506 and cyclosporine acted synergistically in prolonging canine renal allografts and that the vascular changes frequently seen at low doses of FK506 were reduced by additional immunosuppression with a low dose of cyclosporine.