Dietary phosphorus (P) restriction is known to ameliorate secondary hyperparathyroidism in renal failure patients, In early renal failure, this effect may be mediated by an increase in 1,25-(OH)(2)D-3, whereas in advanced renal failure, P restriction can act independent of changes in 1,25-(OH)(2)D-3 and serum ionized calcium (ICa). In this study, we examined the effects of dietary P on serum PTH, PTH mRNA, and parathyroid gland (PTG) hyperplasia in uremic rats, Normal and uremic rats were maintained on a low (0.2%) or high (0.8%) P diet for 2 mo. PTG weight and serum PTH were similar in both groups of normal rats and in uremic rats fed the 0.2% P diet. In contrast, there were significant increases in serum PTH (130+/-25 vs, 35+/-3.5 pg/ml, P < 0.01), PTG weight (1.80+/-0.13 vs, 0.88+/-0.06 mu g/gram of body weight, P < 0.01), and PTG DNA (1.63+/-0.24 vs. 0.94+/-0.07 mu g DNA/gland, P < 0.01) in the uremic rats fed the 0.8% P diet as compared with uremic rats fed the 0.2% P diet, Serum ICa and 1,25-(OH)(2)D-3 were not altered over this range of dietary P, suggesting a direct effect of P on PTG function, We tested this possibility in organ cultures of rat PTGs. While PTH secretion was acutely (30 min) regulated by medium calcium, the effects of medium P were not evident until 3 h, During a 6-h incubation, PTH accumulation was significantly greater in the 2.8 mM P medium than in the 0.2 mM P medium (1,706+/-215 vs, 1,033+/-209 pg/mu g DNA, P < 0.02); the medium ICa was 1.25 mM in both conditions. Medium P did not alter PTH mRNA in this system, but cycloheximide (10 mu g/ml) abolished the effect of P on PTH secretion, Thus, the effect of P is posttranscriptional, affecting PTH at a translational or posttranslational step, Collectively, these in vivo and in vitro results demonstrate a direct action of P on PTG function that is independent of ICa and 1,25-(OH)(2)D-3.