We studied the effect of PRL from two species (bovine and turkey) and GH from two species (bovine and turkey) on 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] production by two whole cell preparations from vitamin D-deficient chick kidneys (slices and tubules). We observed that 8 ng/ml turkey PRL stimulated 1,25(OH)2D3 production by renal tubules and slices. Ovine PRL had a similar effect on 1,25(OH)2D3 production but at higher concentrations. In contrast, neither bovine GH nor turkey GH stimulated 1,25(OH)2D3 production appreciably at doses up to 1000 ng/ml. The effect of PRL on 1,25(OH)2D3 production by renal tubules required a 3-h preincubation, although its effect on 1,25(OH)2D3 production by renal slices was immediate. We conclude that PRL, but not GH, directly stimulates 1,25(OH)2D3 production by the chick kidney.