The results presented here show that a decrease in the concentration of total polyamines, due to a decrease in putrescine and spermine, occurs during oogenesis in Xenopus laevis. The microinjection of spermine or spermidine decreases the hormonal responsiveness (maturation) of the fully-grown oocytes. This effect is synergistic with that already described for the microinjection of casein kinase II (Mulner-Lorillon, O. et al. (1987) Eur. J. Biochem. 171, 107-117), a polyamine dependent enzyme. Therefore a decrease in polyamine concentration, via its effect on endogeneous casein kinase II, could constitute one of the molecular changes required for the acquisition of competence to mature.