1. Introduction The hormonal form of vitamin D la,25(OH),D,, is produced from cholecalciferol (vitamin D3) by C-25 hydroxylation in the liver, followed by C-l hydroxyl- ation in the kidney. The important finding that this hormone is synthesized only in the kidney [ 11, and the demonstration that patients with advanced renal disease are unable to produce la,25(OHhD,, prompted the synthesis of its analogues. The most widely-used analogue in the treatment of vitamin D resistant metabolic bone diseases is la(OH)D, [2-41. The beneficial therapeutic effect of this analogue was shown to be due to its rapid conversion into la,25(OH),D, [S-7]. Thus, patients treated with la(OH)D3 are in fact being treated with a potent calcium mobilizing agent. and the question has arisen therefore whether biological protective mechanisms are operating in order to eliminate toxic responses to such treatments. Previous studies from our laboratories have shown that treatment with loc(OH)D, in chicks results in high accumulation of a non-polar metabolite(s) in the liver [7]. This study describes the characterization of these non-polar metabolites, and the possible role that these metabolites play in the deactivation of la(OH)D3 is discussed.