Toxicity of Mercuric Chloride to the Developing Rat Kidney II. Effect of Increased Dosages on Renal Function in Suckling Pups. It has been demonstrated that the sensitivity to HgCl2 nephrotoxicity increases with maturity in the rat, and that neonates are largely unaffected by a dose of 5 mg/kg. In the present study, immature rat pups were exposed to higher doses of HgCl2 to determine whether this effect was attributable to a quantitiative or qualitative difference in the renal sensitivity to HgCl2. Sprague-Dawley rats were injected with a single dose of 5, 7.5, 10, 12.5, 20, or 30 mg/kg on Postnatal Day 1, 5, 7.5, 10, 12.5, 15, or 20 mg/kg on Day 8; or 6.25, 7.5, 10, or 12.5 mg/kg on Day 15. Renal function was evaluated at 24, 48, and 120 hr after treatment by measuring urine volume, osmolality, urinary pH, and chloride content, the ability to concentrate urine during water deprivation, and the presence of protein, glucose, or hemoglobin in urine. Animals were then killed and their kidneys weighed. A dose of 20 mg/kg was needed to induce mortality in pups treated at 1 day of age, and 15 mg/kg was needed in pups treated at 8 days of age.