Cultures of murine fetal bone explants were used as experimental model for the investigation of drug effects on mesenchymal metabolic processes, particularly the collagen metabolism. As parameters for growth or metabolic processes, resp., the increase in size, the DNA-, uronic acid- and hydroxyproline content of the explants were determined under various modifications of the culture conditions. Addition of ascorbic acid to the culture medium caused in concentrations from 2.5 micrograms/ml an increase of the hydroxyproline content and of the size and in concentrations from 20 or 40 micrograms/ml, resp., also an increase of the DNA- and uronic acid content of the explants. Proline exerted in concentrations up to 100 micrograms/ml no significant influences on the determined parameters and showed in concentrations from 150 micrograms/ml inhibitory effects. Substitution of the horse serum of the culture medium by chick embryonic extract resulted in a decrease of growth and metabolic activity of the explants. Under the influence of an oxygen content of 10% in the gas phase of the culture system all determined parameters with exception of the DNA-content were significantly increased as compared with 20% oxygen content, whereas concentrations of 30% oxygen or more led to reduction of growth and uronic acid content of the explants.