An experimental study was conducted to determine the changes in structure and function of the pig kidney and renal pelvis following intrarenal infusion of bacillus Calmette-Guerin (BCG). Bilateral nephrostomy tubes were inserted in six pigs through a subcostal (flank) retroperitoneal approach. One week later, antegrade pyelograms and renal scans with hippuran I-131 were obtained. The left kidney was then infused weekly for six weeks with two ampules of BCG (Tice strain) dissolved in 75 cc of saline. The right kidney, serving as a control, was infused concomitantly with 75 cc of saline. On week 7, bilateral antegrade pyelograms and renal scans were repeated. Two pigs were sacrificed at four, eight and 12 weeks after completion of BCG therapy. In all pigs, antegrade pyelograms of the left kidney before BCG instillation were identical to those obtained after completion of treatment and to those of the saline infused kidneys. The isotope renal scan in five pigs showed no significant change in image appearance or relative renal plasma flow in the pre-treatment and post-treatment images. In one pig, there was a decrease in the relative clearance of hippuran in the saline infused kidney. In this kidney, an upper pole abscess was found. Microscopic examination of the renal cortex, medulla, pelvis and ureter of the BCG infused kidneys was normal and identical to the saline infused kidneys. The urothelium was intact and no inflammatory changes were noted in the renal cortex or medulla. These results show that direct infusion of BCG into the renal collecting system has no adverse effect on the structure and function of pig kidneys when followed one to three months after treatment.