1. A new micro-method for the estimation of serum calcium requiring only 0.4 C.C. serum is described.
2. Castration causes a persistent fall (17-24 per cent.) in the calcium content of the serum in females of Xenopus laevis as soon as 2 months after operation. In males there is no effect 3 ½ months after castration but a fall (16 per cent.) 6 months after.
3. Removal of the anterior lobe (i.e.. pars anterior and pars tuberalis) of the pituitary leads to a significantly lower calcium level than removal of the entire gland.
4. Injection of antuitrin into completely hypophysectomised animals causes a rise, injection of pituitrin a fall in serum calcium. Injection of antuitrin into normal animals leads to maximal contraction of melanophores on a black background and injection of pituitrin into normal or completely hypophysectomised toads causes maximal expansion on a white background.
5. When the pars tuberalis regenerates the serum calcium returns to normal concomitantly with the reappearance of the white background response, but without regeneration of the ovaries.
6. The influence of the pituitary on ovarian activity and its effect on serum calcium are concomitant but independent activities. It is suggested that the pars tuberalis and the posterior lobe have antagonistic effects on both colour change and serum calcium, and that the pars anterior and the posterior lobe exert antagonistic influences on ovarian activity. It is further suggested that the pituitary directly controls ovarian activity, but that both the pituitary and the gonads control calcium metabolism through some other endocrine gland.