We investigated the effect of a single intravenous dose of Fumonisin $B_1(FB_1$) of rat kidney on the time sequence. Male Sprague-Dawley rats were intravenouslyin jected with FB$_1$at 1.25 mg/kg and were euthanized at 12 hrs, 1, 2, 4, and 6 days after the injection. In $FB_1$ treated rats, serum BUN and creatinine were elevated from 12 hrs. Microscopically, the initial target site was tubules of inner stripe, with mild degenerative and necrotic changes at 12 hrs, but the tubules recovered on day 4. In outer stripe, there were only a few scattered necrotic cells on day 1. These changes became more obvious over the time passed and most severe on day 4. On day 6, regeneration occurred, manifest as hypertrophic, basophilic tubular cells. The dying cells were proved to necrotic cells instead of apoptotic cells by TUNEL. Ultrastructural changes were cytoplasmic vacuole, dilated endoplasmic reticulum, swollen mitochondria, ballooned microvilli of the tubular cell in the outer stripe. These results showed that the renal tubules of outer medulla were the target to $FB_1$-induced nephrotoxicity in the rat. However, initial target was mner stnpe of medulla.