Unfertilized eggs of the echiuroid, Urechis unicinctus, were activated by polyamines, such as putrescine, spermidine and spermine at concentrations above 10 μM. Fertilization membrane elevated and germinal vesicle disappeared in unfertilized eggs kept for several min in sea water containing these polyamines. Following the addition of these polyamines, a decrease of pH value in the egg suspension, occurred in a similar manner as observed following fertilization. Several sec after the addition of polyamines to the egg suspension, the respiratoy rate increased very slightly and the sensitivity of the respiration to 2, 4-dinitrophenol, which was lower in unfertilized eggs than in fertilized eggs, became as high as in fertilized ones. Irregular cleavage occurred in the eggs stimulated by polyamines. The incorporation of [3 H]-deoxyadenosine into DNA was initiated by adding polyamines in the unfertilized eggs preloaded with the isotope. The rate of [3 H]-leucine incorporation into protein in the preloaded unfertilized eggs was also enhanced by polyamines, in almost the same manner as observed following fertilization.