The effect of caffeine was studied on mouse zygotes blocked in the G2 phase of the first cell cycle after X-irradiation. Caffeine (2 mM) effectively reversed the G2 arrest when zygotes were incubated in its presence at the time when first mitosis normally takes place. This effect of caffeine was inhibited by cycloheximide (5 micrograms ml-1). In embryos escaping the G2 arrest the frequencies of chromosome aberrations varied as a function of the time of irradiation, showing a clear relationship with the varying rates of lethality occurring from the morula stage. Blocked zygotes suffered major chromosome damage: however, this did not appear to be the only cause of the G2 arrest. Triploid zygotes were preferentially blocked, suggesting that nuclei contain the target for this X-ray effect. Mouse zygotes are a useful model for studies on G2 arrest and normal cell division regulation.