Calcium, parathyroid hormone, and vitamin D in the "prehypertensive" Dahl salt-sensitive rat. The purpose of this study was to determine if alterations of calcium and calcium regulating hormones precede the onset of NaCl-induced hypertension in the Dahl salt-sensitive (S) rat. After a 5-day balance study, serum ionized calcium, parathyroid hormone (PTH), and 1,25-dihydroxy vitamin D concentrations were measured in Dahl-S and salt-resistant (R) rats that had been maintained on a "normal" (1%) or high (7%) NaCl intake. Blood pressure was higher in Dahl-S than Dahl-R (P less than .01), but was not affected by 5 days of high NaCl. On both NaCl intakes, urine calcium excretion was increased, serum calcium was decreased, and serum PTH and 1,25 dihydroxy vitamin D were increased in Dahl-S compared to Dahl-R (P less than .01). On the high NaCl intake, fecal calcium was greater in Dahl-S than in Dahl-R, and net 5-day calcium balance was less positive in Dahl-S (P less than .05). Thus, alterations of calcium, PTH, and vitamin D precede NaCl-induced hypertension in Dahl-S. These alterations may contribute to the development of hypertension in this animal model. 