A study was made of cyclic guanosine-3+, 5+-monophosphate (cGMP) on protein synthesis in the cell-free system of the thyroid gland. Preincubation of S30 fraction with cGMP led to increase of labeled amino acid incorporation into proteins (by 56% with the cGMP concentration of 1.73 X 10(-5) M, and by 160% with the concentration of 2 X 10(-4)). No stimulating effect was observed on addition of cGMP directly into the incubation medium. The results obtained suggested that protein synthesis was stimulated under the effect of cGMP at the translation level. The same protein fractions [27S, 19S, 12S, 3-8S (I) and 3-8S (II)] were revealed by disc-electrophoresis of soluble proteins synthesized in the cell-free system in which polyribosomes preincubated with cAMP (1.73 X 10(-5) M) were used as in the system devoid of cAMP. Specific radioactivity of thyroglobulin subunits corresponded in experiments with cAMP to that in experiments without cAMP. At the same time specific radioactivity of 19S thyroglobulin was more than doubled. It can be supposed that thyroglobulin subunit synthesis was accelerated under the effect of cAMP, and, as a result, labeled thyroglobulin increased in the system. The data obtained confirmed a hypothesis put forward earlier that cAMP had a direct influence upon the translation process.