The possibility of the involvement of prostaglandins in the mediation and/or modulation of the mitogenic effects of hormones has been discussed. Such a possibility is suggested by the following principal observations: 1. Inhibitors of the prostaglandin synthesis suppress the mitogenic effects of certain hormones, 2. Prostaglandins can be mitogenic. 3. Various mitogenic hormones and non-hormonal mitogens stimulate the prostaglandin synthesis. Hormones are very well known to control the cell proliferation. However, the intracellular mechanisms by which hormones stimulate or suppress the cell growth remain still unclear. It is generally believed that steroid hormones influence the DNA synthesis through a direct interaction of the hormone-receptor complex with nuclear chromatin. On the other hand, the non-steroid hormones are presumed to act via the different intracellular messengers, like cyclic nucleotides, calcium ions and prostaglandins. The present paper attempts to summarize the evidence for the mediation or modulation of the growth-promoting effects of hormones by the latters.