The metabolic activity of pre-implantation blastocysts recovered from delayed mice before and after administration of lead was estimated by measuring their oxygen consumption using a spectrophotometer technique. Normal blastocysts, activated for implantation, increased their oxygen consumption by about 50% compared with inactive, delayed blastocysts. Inactive, delayed blastocysts from lead-injected mice reached the same level of oxygen consumption even without oestrogen administration. Oestrogen given to the lead-injected mice did not further increase the oxygen consumption of their blastocysts. It is concluded that administration of lead to pre-implantation mice resulted in increased metabolic activity of their blastocysts due either to a direct effect of lead or to an indirect effect, caused by changed composition of the uterine secretion.