The projects that will be undertaken are: 1) Activation of delayed implanting mouse blastocyts in vitro appears to involve a significant increase in the rate of embryonic RNA synthesis. It is not clear whether this involves primarily increases in rRNA or mRNA. This will be determined by measuring the effect of alpha-amanitin and actinomycin D or RNA polymerase activity in dormant and active embryos. 2) Mintz has proposed that uterine protease may act as an implantation initiation factor by lysing the zona pellucida and changing blastocyst surface properties causing them to adhere to the uterine wall. We have demonstrated the existence and described the endocrine control of such an enzyme and will now attempt to determine if it has zona lytic properties in vitro. 3) DNA polymerase activity increases as dormant (i.e., delayed implanting) mouse embryos are activated in vitro. It will be determined whether DNA polymerase activity increases when embryos are 'activated' in vitro and if so, whether mRNA synthesis is necessary.