Using uterine lavage performed 5 days after the luteinizing hormone peak, we collected 25 uterine ova from five fertile donors who had had a single, periovular artificial insemination. After examination, all recovered ova were transferred to recipient uteri and resulted in three intrauterine and one tubal pregnancy. Morphologic development ranged from degenerating single-cell ova to mature blastocysts. Ages of the ova at recovery ranged from 93.5 to 130.0 hours postovulation. Mean age of the five blastocysts, 109.1 hours, was not significantly different from the mean age of the 20 less mature ova, 108.1 hours. Neither the mean interval from insemination to recovery for blastocysts nor the mean interval from insemination to ovulation for blastocysts was significantly different from the mean intervals of the other ova. The five blastocysts resulted in intrauterine pregnancies in three recipients. There were no intrauterine pregnancies from the other 20 transfers (p less than 0.004). A transferred 12-cell ovum with degenerating blastomeres was associated with tubal pregnancy in the recipient. The large variability in the state of intrauterine ova observed in the relatively fixed postovulatory interval of this study appears to be due to differences in viability and maturation, not differences in ovum age. Maturational state of the ova at the time of transfer appears to be a significant determinant of the likelihood of ensuring pregnancy.