The establishment of human pregnancies by the use of fertilization in vitro and placing cleaving embryos into the uterus is described. Preovulatory oocytes were aspirated at laparoscopy from the ovaries of patients soon after the beginning of the mid-cycle surge of luteinizing hormone (LH) during the natural cycle. The LH surge was identified by assaying 3-hourly samples of urine, and measurements of oestrogens in 24-hour samples were used to assess follicular growth. The surge of LH was identified in 68 patients and it showed a diurnal rhythm. Preovulatory oocytes were aspirated from most of the patients. Fertilization and cleavage occurred in 34 instances, and 32 embryos were put into the mother via the cervical canal. Four patients became pregnant. There were indications that a diurnal rhythm played a role in establishing cleaving embryos, each of the four pregnancies occurring when the embryos were placed in late evening. Each of the pregnancies resulted from oocytes which were aspirated from their follicles 24 hours or longer after the LH surge began. Details are given of three abortive pregnancies in patients given gonadotrophins to stimulate the maturation of of oocytes used for fertilization in vitro.