The adult rat kidney is an important target organ for sodium fluoride; however, the toxicity of fluoride to the developing kidney is unknown. The study examined renal function following NaF exposure during the first four weeks after birth. Sprague-Dawley rats received a single ip injection of 0, 30, or 48 mg/kg NaF on postnatal day 1, 8, 15 or 29. Alterations in renal function, histology and morphology were determined 24, 38, and 120 hours after exposure. Measurements of renal function included urine volume, osmolality, the ability to concentrate urine during water deprivation, urinary pH and chloride content. Rats were then sacrificed and their kidneys processed for observation by light microscopy. Some minor alterations in renal function were observed in the three youngest age groups after NaF exposure. These changes included decreased body weight after treatment with 30 or 48 mg/kg NaF but increased kidney-body weight ratio in the 49 mg/kg group on day 1; decreased urinary pH in both dose groups after treatment on day 1 or day 8; increased during volume 120 hours after treatment on day 9; and decreased chloride excretion in the 49 mg/kg group 24 hours after injection on day 1. None of these aberrations were severe. In contrast to these results, marked renal toxicity was observed in the post-weaning rats treated on day 29.