Growth hormone (GH) considerably increases the flow of cells into mitosis in rat thymic lymphocyte (thymocyte) populations maintained in vitro. This calcium-dependent, cyclic-AMP-mediated mitogenic action of GH is due to the rapid (within 60 minutes) stimulation of the initiation of deoxyribonucleic acid (DNA) synthesis in a part of the thymocyte population which is poised, or blocked, at the threshold of the DNA-synthetic (S) phase of the cell cycle. A possible mechanism by which GH, through the agency of cyclic AMP, could promote the initiation of DNA synthesis is discussed. 1 Issued as NRCC No. 11814