It has been contended that the metabolism of vitamin D in spontaneously hypertensive rats (SHR) is different from that in Wistar-Kyoto rats (WKY). To investigate this possibility, the plasma concentration of 1,25-dihydroxycholecalciferol (1,25[OH]2D) and several known determinants of its production rate were measured in SHR and WKY given normal and restricted amounts of dietary phosphorus. In 12-week-old male SHR given a normal amount of dietary phosphorus, the mean plasma concentration of 1,25(OH)2D (72 ± 5 pg/ml) was significantly lower than that in agematched WKY (129 ± 6 pg/ml;p< 0.001). The lower plasma concentration of 1,25(OH)2D in the SHR could not be attributed to higher circulating levels of inorganic phosphorus or ionized calcium, lower plasma concentrations of 25-hydroxycholecalciferol, or acidosis. However, in the SHR, urinary excretion of cyclic adenosine 3′,5′-monophosphate (12.5 ± 0.4 nmol/mg creatinine) was significantly lower than that in WKY (15.2 ± 0.3 nmol/mg creatinine;p< 0.001). In both SHR and WKY, restriction of dietary phosphorus for 1 week induced an increase in the plasma concentration of 1,25(OH)2D without affecting blood pressure. The current findings Indicate that in 12-week-old male SHR, 1,25(OH)2D metabolism is different from that in age-matched WKY. The activity of 25-hydroxyvitamin D-la-hydroxylase, however, appears to be at least partially responsive to short-term restriction of dietary phosphorus. In SHR, the activity of 25-hydroxyvitamin D-la-hydroxylase may be lower than that hi WKY, perhaps due in part to some impairment in the renal metabolism of, or responsiveness to, cyclic adenosine 3,5′-monophosphate.