The effect of altering the dietary Ca:P ratio during critical points of growth (based on reproductive and skeletal age) on kidney calcification in female rats was investigated. Groups of weanling animals were fed one of three nutritionally complete but calcium-altered diets (0.25, 0.5 or 1.0 g Ca/100 g diet) from 4 to 12 wk of age (Phase 1). Phosphorus concentration remained constant at 0.4 g/100 g diet resulting in Ca:P molar ratios of 0.48, 0.96 and 1.92, respectively. During Phase 2, the same animals within each diet group were then rerandomized into one of the above diets and fed for an additional 25 wk. Each group contained five rats. The data from the nine treatment groups were analyzed statistically using a two-way ANOVA (Phase 1 dietary Ca level by Phase 2 dietary Ca level). The level of dietary Ca during Phase 1 only exerted a significant influence on kidney Ca accumulation. Rats fed the two lower dietary Ca levels, and hence lower dietary Ca:P molar ratios, during Phase 1 had two- to threefold greater kidney Ca concentration and kidney ash Ca concentration than rats fed the diet with the highest dietary Ca level (1.92 Ca:P molar ratio) during Phase 1, regardless of the Ca intake during Phase 2. In contrast, the dietary Ca:P molar ratio during Phase 2 had little effect either positively or negatively on the kidney Ca concentration that had been established during Phase 1. The results indicate that dietary-induced nephrocalcinosis in female rats is irreversible and is induced primarily before the completion of adolescence (approximately 12 wk of age) in Sprague-Dawley female rats.