Luteinizing hormone (LH), follicle stimulating hormone (FSH), oestradiol and progesterone were measured in plasma from blood samples obtained daily throughout 23 menstrual cycles in a total of 18 normal women. Eight of these women were trying to conceive at the time of investigation and five succeeded. The subjects were divided into those who were not exposed to pregnancy (Group I), those who became pregnant (Group II) and those who were trying to become pregnant but failed to do so (Group III). The mean profiles of each of the four hormones were similar for the three groups studied. Statistical comparisons (using ‘t’ tests) between Groups II and III showed that FSH concentrations were significantly lower (P<0·01) in the late follicular phase (Days —7 to —3 before the LH peak) in Group III. Similarly, progesterone concentrations were significantly lower (P<0·05) during the early luteal phase (Days +3 to +7 after the LH peak) in Group III.

The willing co-operation of all the women who took part in this study is gratefully acknowledged. We are also grateful to Miss L. Brook, Mrs L. Sexton, Mrs D. Cocker, Mrs J. 0. Southall and Mr R. Tovey for skilled technical assistance and to Miss B. McNeill for typing the manuscript. This study was supported in part by the Medical Research Council.