During neonatal testicular development in the rat, events critical for subsequent germ cell development occur that set the stage for fertility later in life. Some gonocytes resume mitotic activity and/or migrate to the surrounding basal lamina, and use of a carefully defined Sertoli cell-gonocyte coculture system indicates that these crucial events occur without added factors or hormones and are hence likely to depend on interaction with adjacent Sertoli cells. Coupling of the Kit receptor protein on gonocytes to stem cell factor from Sertoli cells is vital for successful migration by gonocytes, as antagonism of the former suppresses and addition of the latter stimulates gonocyte migration. During the neonatal period, intercellular adhesion is modified in a developmental manner such that neural cell adhesion molecule (NCAM) is the main adhesive molecule expressed and functioning at birth, with a progressive decline as development proceeds. This decline in NCAM expression is supported by the addition of exogenous 3,3,5-triiodothyronine in vitro, and because this factor is recognized as supporting Sertoli cell differentiation, it seems likely that changing intercellular adhesion is a function of progressive development of Sertoli cells. Other avenues whereby maturing testicular cells influence each other doubtless exist, including secretion of growth factors and other peptides and developmentally important changes in the makeup of the extracellular matrix, which Sertoli cells and gonocytes contact. Continued investigation in these areas will be very valuable in enlarging our understanding of how neonatal testicular development provides the basis for successful spermatogenesis.