Cellular accumulation and toxicity of inorganic mercury were studied in suspensions (1 mg protein/ml buffer) of proximal tubular segments isolated from the kidneys of rabbits. Mercuric chloride containing trace amounts of radiolabeled inorganic mercury (203Hg2+) was added to the buffer to produce a concentration of inorganic mercury ranging from 0.1 to 10 microM. Significant release of lactate dehydrogenase (LDH) and significant decreases in oxygen consumption (QO2), which were used as indices of cellular injury, were detected only when the tubules were in the presence of 10 microM inorganic mercury. At this concentration of inorganic mercury, cellular release of LDH increased and QO2 decreased significantly between the 1st and 4th hr of exposure, by which time most of the proximal tubular cells were necrotic. Maximal cellular content of inorganic mercury was attained within the first 5 min of exposure, during which time nearly 70% of the inorganic mercury in the bath was removed. Accumulation of mercury was more gradual when the tubules were exposed to 0.1 microM inorganic mercury. Addition of 40 microM glutathione, cysteine, or bovine serum albumin to the bath provided the segments of the proximal tubule with complete protection from the toxic effects of 10 microM inorganic mercury. The rate of uptake of inorganic mercury was also significantly decreased. By the end of 4 hr of exposure only about 30% of the content of mercury in the bath was abstracted. These findings indicate that isolated segments of proximal tubules take up inorganic mercury very rapidly and subsequently become intoxicated. They also show that when compounds containing free sulfhydryl groups are in the presence of inorganic mercury in the bath, the rate of uptake of inorganic mercury is significantly decreased and the tubules are provided protection from the toxic effects of the inorganic mercury.