Serum MSH levels and pituitary MSH content: their fluctuation during pregnancy in the rat.

MSH was measured by a biological assay in the pituitary and serum of the rat during the whole term of pregnancy. The peptide showed changes in early pregnancy in serum, during the middle of pregnancy values of serum MSH became negative, and in late pregnancy it was possible to find high values during days 18 and 19 before delivery. Comparison between the amount of pituitary MSH and that in serum seems to show that between days 10 and 15 there is practically no, or just a litte, MSH synthesis, since the MSH values in serum are negative although the content of pituitary MSH is also low. A relation between the results found by other authors and that found by us was analyzed.