Using cultured normal rat chondrocytes we have investigated 1) the effects of somatomedin and insulin on glycosaminoglycan (GAG) synthesis and 2) somatomedin and insulin binding sites. We confirmed that somatomedin and insulin stimulate GAG synthesis in normal rat chondrocytes. The maximal responses of somatomedin and insulin in GAG synthesis were the same, but the stimulation of GAG synthesis by maximally effective concentrations of insulin plus somatomedin was not cumulative. Cultured rat chondrocytes had binding sites for somatomedin and insulin, and the binding was displaced by both somatomedin and insulin. Somatomedin is much more effective than even large amounts of unlabelled insulin in displacing both somatomedin and insulin binding. Anti-insulin receptor IgG, which inhibits insulin binding to human placental membrane, did not affect GAG synthesis stimulated by insulin nor did it inhibit insulin binding to chondrocytes. Somatomedin used for GAG synthesis and displacement was the partially purified somatomedin A with a biological activity of 80 U/mg. Therefore the possibility that substances other than somatomedin A, which was contaminated in this preparation, affected this result could not be excluded completely. However, these results suggest that somatomedin and insulin act on normal rat chondrocytes through a somatomedin receptor.