The serum calcium of rabbitsis reduced by the following pituitary preparations:a crude pituitary extract, pep tidesI and II, oxycellulose-purified corticotropin,a-MSH and β-MSH. Following subcutaneousinjection the serum calcium reaches minimalvalues in approximately 2 hr and returns tonormal after 4 hr. These factors also producean increase in serum free fatty acids; fastingpotentiates both effects, suggesting that the 2effects may be related. The hypocalcemiaoccurs despite the injection of large amountsof parathyroid extract. In dogs or rats thesepeptides have little or no effect on either theserum calcium or the free fatty acids. (Endocrinology75: 692, 1964)