Abstract When paragonia were incubated for 8 hr in simplified Robb's medium a continuous increase in incorporation of 14 C-amino acids into TCA precipitable proteins was found. The increase is linear over the first 2 hr. Copulation provides the stimulus for an alteration in rate of in vitro incorporation: a rapid and clear increase occurs. A similar increase in rate of incorporation of 14 C-uridine into RNA and 3 H-ethanolamine into ‘paragonial substance’ could be found. The pattern of incorporation of 14 C-amino acids into proteins, as determined by polyacrylamide gel electrophoresis, remains qualitatively unaltered. Results from transplantation experiments of single paragonia suggest that the stimulation, at least in part, is neuronal or humoral mediated. In vitro incubations with actinomycin D, under conditions of 90% inhibition, resulted in an increased rate of incorporation of 14 C-amino acids into proteins.