The biosynthesis of proteoglycans was studied in organ cultures of differentiating metanephric mesenchymes. When triggered by a contact-mediated inductive interaction, this tissue undergoes transition from a mesenchyme to an epithelium. In the present study, proteoglycans were extracted by guanidinium hydrochloride in the presence of protease inhibitors. We found that, as a response to induction, the differentiating mesenchyme begins to synthesize large size proteoglycans with an apparent molecular weight (MW) of 1 X 10(6) D. The major glycosaminoglycans detected were chondroitin sulfates. Heparan sulfate proteoglycans were also detected, constituting 20% of the proteoglycans. An inhibitor of glucosamine synthesis, 6-diazo-5-oxo-norleucine (DON) was found to inhibit glycosaminoglycan synthesis by approx. 60%, and the size of the proteoglycans was also diminished. Our studies suggest that the transition of the mesenchyme to epithelium is associated with initiation of synthesis of large size proteoglycans.