The synthesis of extracellular matrix of mesoglea has been studied using autoradiography with labeled amino acids (³H-leucine, ³H-proline, ³H-lysine) and ³H-glucose in young and mature medusae of Aurelia aurita. An intensive secretion of proteins and polysaccharides into mesoglea was shown both in young and mature medusae. Collagen is secreted almost exlusively by epithelium of ex- and subumbrella, the gradient of ³H-proline distribution in mesoglea being preserved for two days. On the other hand, the gradient of distribution of other amino acids disappears in 7-13 h. This fact may reflect the synthesis of other, non-collageneous proteins, rapidly diffusing through the mesogleal matrix. In comparison with other tissues the ameboid cells of mesoglea show a selective uptake of ³H-lysine. The data obtained in this work, together with the results of our previous studies (Napara et al., 1994, 1995), allow to suppose that the mesogleal cells of A. aurita may participate in the synthesis of non-collageneous, lysine-rich proteins of the extracellular matrix of mesoglea.