BACKGROUND Dihydrotachysterol(2), a licensed pharmaceutical, is hydroxylated to 25-hydroxydihydrotachysterol(2) (25(OH)DHT(2)) and 1 alpha,25-dihydroxydihydrotachysterol(2) (1 alpha,25(OH)(2)DHT(2)) in man. We have compared the biological activity of these metabolites with calcitriol and the 'non-calcaemic' analogue, 22-oxacalcitriol (OCT) in bovine parathyroid cell cultures and in rats. METHODS The effect of each sterol on parathyroid hormone (PTH) secreted by primary bovine parathyroid cells was measured. High-performance liquid chromotography and gas chromotography-mass spectrometry were used to investigate in vitro 25(OH)DHT(2) metabolism. Rats were given a single intraperitoneal injection or five daily injections of each sterol, and changes in ionized calcium and PTH were measured. RESULTS In vitro, all sterols suppressed PTH significantly. Calcitriol and OCT were of similar potency, but 1 alpha, 25(OH)(2)DHT(2) and 25(OH)DHT(2) required higher concentrations to suppress PTH equally. We were unable to detect metabolism of 25(OH)DHT(2) to 1 alpha,25(OH)(2)DHT(2) in vitro. In rats, a single dose of 0.5 microg/rat of calcitriol increased ionized calcium at 30 and 40 h (statistically significant at 48 h). 50 microg of OCT and 1 alpha,25(OH)(2)DHT(2) did not cause significant hypercalcaemia at 48 h, although 1 alpha,25(OH)(2)DHT(2) caused hypercalcaemia at 30 h. In contrast, 50 microg of 25(OH)DHT(2) caused hypercalcaemia at 48 h but not at 30 h. Five daily doses of 0.001 microg/rat of calcitriol caused a significant rise in calcium and a 50% fall in PTH. OCT and 1 alpha,25(OH)(2)DHT(2) at 0.025 and 0.5 microg/rat respectively caused similar suppression of PTH but without hypercalcaemia. CONCLUSIONS 1 alpha,25(OH)(2)DHT(2) and 25(OH)DHT(2) are potent suppressors of PTH in vitro and in vivo. 25(OH)DHT(2) may be active by virtue of its pseudo-1 alpha-hydroxyl group. Hypercalcaemia caused by a single dose of 1 alpha,25(OH)(2)DHT(2) appeared to be more transient than calcitriol. Five daily doses of 1 alpha, 25(OH)(2)DHT(2) and OCT could achieve 50% suppression of PTH without significant increments in ionized calcium. In contrast, suppression of PTH by calcitriol was associated with significant increments in ionized calcium. These data suggest that like OCT, 1 alpha, 25(OH)(2)DHT(2) can dissociate calcaemic actions from parathyroid-suppressing actions in a manner that may be therapeutically useful.