The functional, transplanted, autonomous rat pituitary â€oemammotropic―tumor (Furth, MtT.F4) was compared with normal rat pituitary for capacity to elaborate prolactin in vivo and in vitro. The prolactin content of transplanted anterior pitui tary (AP) and MtT.F4, as measured by the intradermal pigeon crop technic, was 1.6 and 4.8 per cent, respectively, of that present in the AP in situ. Considerable amounts of prolactin were detected in the blood plasma of rats bearing an MtT.F4, the levels

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... ing higher in rats with larger than in rats with smaller tumors. Prolactin activity could not be detected in the blood plasma of normally cycling, estrogen-treated, pregnant, or lactating rats, or in rats bearing a single AP transplant underneath the kidney capsule. The amount of prolactin released into the medium by \ItT.F4 tissue during 3 days of culture was 8â€"31times greater than that present initially, indicating active synthesis and release of prolactin by the tumor. These results show that the MtT.F4, like nontumorous AP transplants removed from hypothalamic inhibition, syn thesizes prolactin and releases it at a rapid rate, leaving little in the tissue itself.