Twenty Jersey cows were fed one of four prepartal diets: a)low calcium, low phosphorus (LCLP); b) low calcium, high phosphorus (LCLP); c) high calcium, low phosphorus (HCLP); or d)high calcium, high phosphorus (HCHP). Diets were fed for about 4 weeks prepartum. Blood samples were taken periodically, and the collected plasma analyzed for concentrations of calcium, phosphorus, hydroxyproline and 1,25 dihydroxyergocalciferol plus 1,25-dihydroxycholecalciferol (1,25-(OH)2D). Cows fed the LCLP and LCHP diets, when compared to cows fed the HCLP diet, had: a) greater concentrations of plasma 1,25-(OH)2D and hydroxyproline prepartum; b) greater plasma calcium concentrations at parturition; and c) less incidence (0 versus 4 cases) of parturient paresis. Thus, low calcium diets, regardless of dietary phosphorus intake, seemed to activate calcium homeostatic mechanisms before parturition by stimulating both bone and gut. Cows fed the HCLP diet had greater plasma calcium concentrations at parturition than did cows fed the HCHP, even though there was no measurable effect on plasma 1,25-(OH)2D and hydroxyproline concentrations during the prepartal period. It seems possible that the beneficial effect of low dietary phosphorus, when dietary calcium is high, may be a result of a prepartal increase in efficiency of absorption of calcium and phosphorus from the gut caused by increased binding of 1,25-(OH)2D to intestinal receptors.