In 30 normal subjects, the mean (+/- SEM) plasma concentration of PRL was 5.90 +/- 0.40 ng/ml and that of PTH was 0.51 +/- 0.03 ng/ml. There was no significant difference in plasma hormone levels according to age or sex. Ten cases of primary hyperparathyroidism showed PRL concentrations (8.90 +/- 1.80 ng/ml) significantly (P less than 0.01) higher than those of the normal subjects. After adenomectomy, the PRL concentration decreased (5.35 +/- 0.50 ng/ml). However, this decrease was only significant in the 5 of 10 patients who had preoperative plasma PRL levels of 10 ng/ml or more (P less than 0.01). The increase in PRL concentration in 10 cases of secondary hyperparathyroidism with normal glomerular function was also significant (14.25 +/- 3.9 ng/ml; P less than 0.001). Fourteen patients with prolactinoma showed PTH plasma levels (1.25 +/- 0.15 ng/ml) significantly higher than those of normal subjects (P less than 0.001). Eight of the 14 patients received 7.5 mg/24 h of bromocriptine for 3 months; their mean plasma PTH level decreased significantly from 1.60 +/- 0.35 to 0.50 +/- 0.11 ng/ml (P less than 0.01). In 9 cases of secondary hyperprolactinemia, the increase in PTH (0.80 +/- 0.16 ng/ml) was significant compared to the plasma PTH levels in the normal group (P less than 0.05). These results show that an excess of plasma PRL is associated with an excess of plasma PTH and vice versa. The mechanisms of these relationships remain unclear.