The steroid hormone 1 alpha,25(OH)(2)-vitamin D(3) [1 alpha,25(OH)(2)D(3)] mediates through its widely distributed nuclear receptor (VDR(nuc)) regulation of gene transcription (genomic responses) and through a putative membrane receptor (VDR(mem)) a variety of rapid responses. Rapid responses studied in our laboratories include opening of voltage-gated calcium and chloride channels in ROS 17/2.8 osteoblast cells, activation of MAP-kinase in human leukemia NB4 cells and chick intestinal cells, release of insulin by rat pancreatic beta-cells, and in chick duodena transcaltachia (the rapid hormonal stimulation of intestinal Ca(2+) transport). 1 alpha,25(OH)(2)D(3) is conformationally flexible (side chain, seco B-ring and A-ring) and accordingly is able to generate a large array of different shapes to serve as ligands for available receptors (VDR(nuc) and VDR(mem)) in the vitamin D endocrine system. Our laboratories have utilized a number of conformationally restricted analogs of 1 alpha,25(OH)(2)D(3) (from a library of several hundred analogs) to evaluate the preferred shape of the ligands for rapid and genomic responses. The determination of the X-ray structure of the 1 alpha,25(OH)(2)D(3)-occupied VDR(nuc) revealed that the preferred ligand shape was a twisted 6-s-trans bowl shape [Molecular Cell 5 (2000) 173-179]. Optimal agonists for genomic responses include 1 alpha,25(OH)(2)D(3) and other side chain conformationally flexible analogs such as 20-epi-1 alpha,25(OH)(2)D(3) [approximately equal to 200-500-fold more potent than 1 alpha,25(OH)(2)D(3)] and 21-(3'-hydroxy-3-methylbutyl)-1 alpha,25(OH)(2)D(3) [an analog with two side chains] all which can achieve the preferred VDR(nuc) shape. In contrast, rapid responses require a 6-s-cis shape of the agonist ligand such as can be achieved by the natural hormone 1 alpha,25(OH)(2)D(3) or by analogs permanently locked in the 6-s-cis shape such as 1 alpha,25(OH)(2)lumisterol(3) or 1 alpha,25(OH)(2)-7-dehydrocholesterol. Additionally, we have discovered analogs that are specific in their antagonist properties for either rapid or genomic responses. Thus, 1 beta,25(OH)(2)D(3) is an antagonist of only rapid responses [via the VDR(mem)], while 23S-25-dehydro-1 alpha,25(OH)D(3)-26,23-lactone is an antagonist of only nuclear responses [via the VDR(nuc)]. In conclusion, we have presented evidence that 1 alpha,25(OH)(2)D(3) mediated rapid response and genomic response signal transduction pathways utilize differing shapes of ligand, both as agonists and antagonists.