The roles of prolactin and thyrotrophin (TSH) in the regulation of glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) activities in rat mammary glands were investigated by the administration of thyroid hormonereleasing hormone (TRH), bromocriptine, prolactin, TSH and triiodo-1-thyronine (T<sub>3</sub>). TRH failed to induce changes in the activities of these enzymes in glands from intact animals but the releasing hormone significantly increased the activities of both the enzymes in the glands of ovariectomized and adrenalectomized animals. Bromocriptine administration had no effect on the activities in both, intact and ovariectomized-adrenalectomized animals. Administration of ovine prolactin to hypophysectomized rats did not affect the activities of these enzymes. On the other hand treatment with TSH resulted in significant increases in the activities. Similarly, administration of T<sub>3</sub> to these animals resulted in changes similar to those observed after TSH administration. Combined administration of prolactin and TSH showed that prolactin is capable of partially inhibiting the TSH-induced increases. It is concluded that TSH is involved in the channelling of substrates into the pathways of nucleic acid synthesis and prolactin probably plays a regulatory role in this process.