The effect of hypocalcemia following parathyroidectomy (PTX) on growth hormone (GH) secretion was investigated in unrestrained, unanesthetized male rats bearing chronically implanted indwelling cannulae. During a 6-hour period, starting at about 10 a.m., control rats with a serum calcium (Ca) value of 8.11 ± 0.38 mg/dl (mean ± SEM) 2 weeks after sham-operation showed secretory bursts of GH similar to those observed in conscious intact rats. Under hypocalcemia of 4.88 ± 0.32 mg/dl 2 weeks after PTX, GH secretory episodes were completely suppressed throughout the study. Plasma prolactin (PRL) levels were also decreased in PTX rats as compared with those of sham-operated rats. Daily food intake and body weight gain as well as serum T<sub>4</sub> levels in PTX rats were not different from those of sham-operated and intact rats. Pituitary GH content of PTX rats was significantly lower than that of sham-operated and control rats. Pulsatile GH secretion was partially restored in PTX rats by raising serum Ca to 8.43 ± 0.27 mg/dl through feeding with high Ca diet containing 7% Ca. Immediately after intravenous injection of antisomatostatin sheep serum, pulsatile GH surges recovered in PTX rats despite hypocalcemia of 4.48 ± 0.74 mg/dl. The mean plasma 6-hour GH levels were significantly higher than those of normal sheep-serum-treated PTX rats (p < 0.001). These findings suggest that the episodic release of GH is suppressed in hypocalcemic rats after PTX, at least partially via circulating endogenous somatostatin.