The renal disposition and the intrarenal distribution of albumin and mercury were studied simultaneously in rats co-injected with a 0.5-mumol/kg dose of albumin and a 0.25-mumol/kg dose of inorganic mercury at 2, 5, 30, and 180 min after injection. These studies were carried out to test the hypothesis that one of the mechanisms involved in the renal tubular uptake of inorganic mercury is cotransport with albumin. By the end of the first 2 min after injection, the ratio of inorganic mercury to albumin in the renal cortex and outer stripe of the outer medulla was approximately 2.6 and 1.6, respectively. Both the cortex and outer stripe contain segments of the proximal tubule, and it is these segments that have been shown to be principally involved in the renal tubular uptake of both albumin and inorganic mercury. The ratio increased slightly in these two zones after 5 and 20 min after injection. These data demonstrate that there is a relatively close relationship in the renal content of inorganic mercury and albumin during the early minutes after coinjection of inorganic mercury and albumin. However, the ratios are significantly greater than the ratio of inorganic mercury to albumin in the injection solution, which was 0.5. After 180 min following co-injection, the ratio increased to about 38 in the cortex and 15 in the outer stripe. This increase in the ratio is probably related to the metabolism of albumin. Based on the ratios of inorganic mercury to albumin in the renal cortex and outer stripe of the outer medulla, it appears that some proximal tubular uptake of inorganic mercury occurs by mechanisms other than endocytotic cotransport of inorganic mercury with albumin. However, since the ratios were small during the early times after injection, cotransport of inorganic mercury with albumin cannot be excluded as one of the mechanisms involved in the proximal tubular uptake of inorganic mercury.