Mouse embryos were treated with 5-bromodeoxyuridine (BrdUrd) for 24 hours at various preimplantation stages to determine its effect on early postimplantation development. The inhibitory effect of BrdUrd (10(-7) to 10(-6) M) on trophoblast outgrowth and inner cell mass (ICM) development was least severe when embryos were treated at the 2-cell stage and was most severe when they were treated at the morula stage. When embryos were treated at the blastocyst stage, the inhibitory effect on trophoblast outgrowth decreased, but that on ICM development remained severe. The severity of inhibition, particularly that on ICM development, appeared to be related to decreased cell numbers in BrdUrd-treated embryos. However, increasing the cell number by aggregating two ICM's isolated from BrdUrd-treated blastocysts did not increase their chance of survival or of forming two primary germ layers. This indicates that the decrease in cell numbers alone is not the cause of the failure of BrdUrd-treated embryos to develop. The mechanism of BrdUrd inhibition was studied by adding thymidine or deoxycytidine during BrdUrd treatments. A 10-fold excess of thymidine completely protected embryos from the inhibitory effect of BrdUrd. A 10-fold excess of deoxycytidine was less effective. Autoradiography indicated that both thymidine and deoxycytidine protected embryos by interfering with the incorporation of BrdUrd into the DNA.