Collagen metabolism was investigated in the fibrotic tissue which developed in pig thigh muscle 6 to 15 months after acute gamma irradiation. During this period, total collagen deposits in the fibrotic tissue increased 10-fold compared to the healthy muscle tissue. These deposits were composed mainly of type I and III collagen, and the type I/type III ratio was lower in the fibrotic than in the muscle tissue. Small pieces of both fibrotic and muscle tissue were incubated with [14C]proline. The [14C]hydroxyproline content of the fibrotic tissue reflected large concomitant increases in the synthesis of total collagen, mainly of types I and III, which rose 14- and 17-fold, respectively. Similarly, the level of type I and type III procollagen mRNAs rose 9- and 5-fold, respectively, in the fibrotic tissue versus the muscle tissue. These results suggest that procollagen gene transcription or RNA maturation in the cell nuclei is activated in the fibrotic tissue. The possibility that such activation is due to the long-term inflammatory state of this tissue is discussed.