The pulsatility of LH secretion has been known only since 1970. The pulsatility of progesterone secretion has now also been shown. The frequency and amplitude of these pulses vary during the three phases of the luteal period. The first or early luteal (EL) phase lasts four days starting from the peak of LH. There is no pulsatility of progesterone during EL. The frequency of LH pulsatility is 103 +/- 8 minutes. The second or mid luteal (ML) phases, from +6 to +9 days after the peak of LH, is characterised by pulses of LH and of progesterone, most often in positive crossover correlation and with 7 to 14 pulses per 24 hours. The frequency of LH pulsatility is 166 +/- 33 minutes according to Filicori. The amplitude of LH secretion is 4 to 40 ng/ml. The third or late luteal (LL) phase, occurring at +11 days before the menstrual period, has the same characteristics in terms of quality but shows variations in frequency and amplitude towards a decrease. Thus the amplitude of progesterone secretion is 2.8 to 9 ng/ml (Veldhuis). In the opinion of clinicians, the corpus luteum is independent up to LH+ 5 days. The injection of hCG during this time is ineffective. A single progesterone level is sufficient if the result is 10 ng/ml or more. The problem remains complex concerning results below 10 ng/ml. The solution suggested for the present is that described by Olive: during ML and/or LL, three samples in the morning at hourly intervals and two endometrial biopsies during two consecutive cycles.