By means of radioautography, 35S methionine was used as an index of protein synthesis in cleaving mouse ova. Limited amounts of 35S methionine were incorporated in early cleavage stages. However, the normal blastocyst showed intense reactivity shortly before implantation (Day 5 of pregnancy). When a suckling stimulus was used to delay nidation, the delayed implanting blastocysts did not incorporate 35S methionine for at least as late as Day 9 of concurrent pregnancy and lactation. However, when the young were removed on Day 8 of lactation, the blastocysts incorporated 35S methionine 24 hr later to the same extent as normal Day 5 blastocysts. This indicates that the prolonged uterine sojourn of the delayed implanting blastocyst is accompanied by modifications in metabolic activity so that the egg more closely resembles the pattern of development during the first 4 days of pregnancy than the normal blastocyst.