A pre-LH peak rise of progesterone in peripheral blood has been found in 13 normal cycling women whose ovulation was confirmed by biopsy of the corpus luteum through serial determination of progesterone and LH performed every 8 h during the periovulatory period. The progesterone rise began as an average 22 h (16-40 h) prior to the LH peak. The maximal preovulatory rise took place 9.6 h (0-24 h) before the LH zenith, remaining low for approximately 17 h when an abrupt rise of progesterone took place. The progesterone peak was detected in the morning samples in 11 of 13 patients studied. The progesterone rise was always followed by an LH peak and the highest peak of progesterone was trailed by the highest LH peak in all the patients except one.

The ultrastructure of a 4-cell human embryo undergoing development in vivo is described. The embryo recovered from the Fallopian tube 130 h after a single episode of intercourse and 70 h after the luteinizing hormone peak in plasma, was partially surrounded by cumulus cells. The blastomeres, of even size, were nucleated and had a similar distribution of cytoplasmic organelles. No signs of abnormality or cellular degeneration were observed. Transmission electron microscopy of serial sections revealed the presence of primitive desmosomes between cells, abundant vesicles within the blebs of the outer sheet of the nuclear membrane, blebbing of the inner sheet of the nuclear membrane and close association between the inner sheet of the nuclear membrane and the intranuclear annulate lamellae. Nucleolar precursors lacking the structural organization of mature nucleoli were also found. Similarities and differences between this and other preimplantation human embryos reported earlier are analysed.

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