SUMMARY Experiments were carried out to assess the possible involvement of cyclic AMP in the regulation of masculine sexual behaviour by testosterone in the laboratory rat. Doses of testosterone propionate which were ineffective in maintaining or inducing male sexual behaviour were potentiated by concurrent administration of theophylline. Since removal of the adrenal glands had no effect on this potentiation, the possibility that theophylline increased adrenal androgen secretion and thereby influenced sexual behaviour, can be ruled out. A major effect of theophylline is to increase levels of cyclic AMP by inhibiting the enzyme which inactivates this nucleotide. These results suggest that cyclic AMP may play a mediating role in the regulation of masculine sexual behaviour by testosterone.