Abstract. Bone organ culture makes it possible to observe the direct influence of hormones on bone cells. We studied the effect of growth hormone in vitro on embryonal rat tibiae during culture for 7 days, functionally by measuring the levels of alkaline phosphatase in the culture medium, and morphologically by means of semithin sections and electron microscopic examination. Since growth hormone (GH) is supposed to exert an indirect effect on bone cells, somatomedin-C/insulin-like growth factor I (SM-C/IGF I) as a possible mediator was also measured radioimmunologically in the culture medium. In the controls alkaline phosphatase levels showed a continuous increase up to the 7th day which was significantly higher in the presence of GH. There was also a significantly enhanced increase of SM-C/IGF I in the presence of GH during culture in comparison to the controls. Evidently IGF I is produced locally in bone and mediates the effect of GH on bone formation.

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