Twenty-one aged Jersey cows were fed a high calcium diet prior to parturition to predispose them to parturient paresis. Eleven cows were implanted subcutaneously with pellets containing 24F-1,25-dihydroxyvitamin D3 7 d before the expected date of parturition and thereafter at 7-d intervals until parturition. Ten cows were left untreated to serve as controls. Incidence of parturient paresis among control animals was 80% (8/10). Treatment reduced the incidence of parturient paresis to 9% (1/11). The mean observed plasma calcium concentration nadir of implanted cows was 6.61 +/- .40 mg/dl, which was significantly greater than the plasma calcium nadir of 4.45 +/- .39 mg/dl observed in the control cows. Two steers were implanted with and three nonpregnant, nonlactating cows received intramuscular injections of 24F-1,25 dihydroxyvitamin D3 to contrast circulating plasma concentrations achieved by the two routes. Intramuscular injection and implantation resulted in plasma 24F-1,25 dihydroxyvitamin D3 concentrations above 300 pg/ml for the first 48 h after administration. From d 4 until d 11 after administration, plasma concentration was maintained between 164 and 89 pg/ml in the implanted steers. Plasma concentration was undetectable 7 d after an intramuscular injection. These data indicate that, with refinement, sustained release of 24F-1,25 dihydroxyvitamin D3 can be successfully used to reduce the incidence of parturient paresis.