A primary chick kidney cell culture is described, capable of forming 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], 24,25-dihydroxyvitamin D3 [24,25(OH)2D3], and 1,24,25-trihydroxyvitamin D3 [1,24,25(OH)3D3] over several days. The apparent Km values were 0.125 microM for the 1-hydroxylase and 2.1 microM for the 24-hydroxylase. Exogenous 1,25(OH)2D3 decreased 1-hydroxylase and increased 24-hydroxylase within 4 h. 24,25(OH)2D3 produced similar effects, but only in the absence of fetal calf serum. R and S isomers of 1,24,25(OH)3D3 were about fives times less active than 1,25(OH)2D3. Bovine parathyroid hormone stimulated the 1- and reduced the 24-hydroxylase in 6 h, but this only occurred in cultures either previously treated with 1,25(OH)2D3 and EGTA to lower Ca to 0.8 mM or in cultures grown in the presence of 25-hydroxyvitamin D3 (25(OH)D3). Under the latter condition, the sensitivity to bovine parathyroid hormone was enhanced, 0.04 U/ml producing a maximum response. Synthetic aminoterminal tetratriacontapeptide (1-34) human parathyroid hormone was equally effective. In the absence of D metabolites, estradiol for 6 h produced a dose-dependent inhibition of the 1-hydroxylase, but no change in the 24-hydroxylase. Progesterone, testosterone, and corticosterone had no significant effect. In cultures grown in the presence of 25(OH)D3 no reproducible effects were obtained with either 1 microM estradiol or 1 microM testosterone, alone or in combination, but 5 microM corticosterone decreased the 1- and increased the 24-hydroxylase. Changes in Ca and P concentrations of the medium as well as addition of ethane-l-hydroxy-1, 1-diphosphate for 48 h did not affect any of the hydroxylase activities. The modulation of the hydroxylase activities by vitamin D3 metabolites and parathyroid hormone suggests that these factors regulate the renal hydroxylase by direct actions, whereas it would appear that ethane-1-hydroxy-1,1-diphosphate, Ca, P, and steroid may exert their influence indirectly.