A bnormalities of intestinal calcium absorption and the vitamin D axis in the spontaneously hypertensive rat (SHR) are controversial. The present report documents a reduction in circulating 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in the 12-14-wk-old male SHR with evidence of its functional significance. Both plasma 1,25(OH)2D3 and mucosa-to-serosa duodenal calcium flux (Jm-s), measured by the Ussing chamber, were significantly lower (approximately 60% of value in Wistar-Kyoto rats [WKY]) in SHR on both normal (1%) and low (0.1%) calcium diets than in corresponding control WKY. Low dietary calcium increased both 1,25(OH)2D3 and Jm-s by approximately 80% in SHR and WKY, with levels of both parameters rising in the SHR to levels found in the WKY under baseline conditions. The latter fact suggests the improbability of intestinal resistance to the action of 1,25(OH)2D3 in the SHR. Plasma 25-hydroxyvitamin D3 (25(OH)D3) was not significantly different between the strains. Intraperitoneal 1,25(OH)2D3 increased Jm-s in 12-14-wk-old SHR to levels observed in equivalent WKY. In 20-24-wk-old SHR, calcium deprivation was associated with significantly reduced Jm-s compared with equivalent WKY. Bone density and bone calcium content in 20-30-wk-old SHR were significantly reduced. In summary, we provide evidence that the SHR was unable to sustain appropriate circulating levels of 1,25(OH)2D3, an impairment which resulted in reduced duodenal calcium absorption.