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1. Explants of rat adenohypophyses were incubated in medium 199, Earle's saline, or modified Earle's saline (K-Earle's) with different calcium, magnesium, sodium and potassium concentrations for 4-6 hr at 37° C in 5% CO₂ in O₂.

2. The amount of prolactin and growth hormone in explants and in medium was estimated by disk electrophoresis and densitometry.

3. Low calcium inhibited prolactin secretion by 40-60%. Low magnesium and slightly increased (5x) magnesium (4·1 mM) did not influence prolactin secretion in either 1·8 mM or low calcium, but 10 and 20 mM magnesium inhibited secretion by 50 and 70%, respectively. Inhibition of prolactin secretion by high magnesium was reversible.

4. Decreasing sodium by 80% had no effect on prolactin secretion, but low sodium inhibited the sustained, although not acute, secretion of this hormone.

5. Low potassium did not affect the secretion of either prolactin or growth hormone. Increased potassium (32 or 54 mM) slightly stimulated prolactin secretion by adenohypophyses of males but had no effect on the secretion of this hormone by female glands.

6. In 1·8 mM calcium, growth hormone secretion by pituitaries of males and females was markedly stimulated by increased potassium, but it was not appreciably affected by high potasium in low calcium. Increased potassium did not stimulate growth hormone secretion in medium containing low sodium with normal calcium.

7. Secretion of prolactin and growth hormone by rat adenohypophyses is dependent upon calcium, but only growth hormone secretion is markedly stimulated by increased potassium. The difference in responsiveness to increased potassium, between the somatotrophs and lactotrophs, may be related to the predominant mode of hypothalamic control of these hormones.