Two cycles each with full endocrine profiles from 17 subjects were examined for within-subject (between cycles) and between-subject variability (one way analysis of variance) and for the degree of correlation between pairs of cycles (regression analysis, Pearson's correlation coefficient). LH concentrations were significantly correlated within subjects for the mid cycle (P less than 0.0001) and luteal phases (P less than 0.05) whereas FSH concentrations were significantly correlated throughout the cycle (P less than 0.01). Oestradiol, however, was only weakly correlated (P less than 0.05) and then only during mid-follicular and luteal phases. Progesterone concentrations were highly significantly correlated (P less than 0.0001) during the luteal phase as were both follicular (P less than 0.01) and luteal phase length (P less than 0.0001) and prolactin concentrations (P less than 0.001). All the reproductive hormones showed less variation within an individual than between individuals. This was particularly marked for FSH and prolactin (throughout the cycle) LH (during the mid-cycle surge) and progesterone concentrations (luteal phase). It is likely that the peripheral plasma levels of these hormones are characteristic of the individual and do accurately reflect the state of equilibrium existing between all of the physiological regulatory mechanisms. Further, the current practice of describing endocrine profiles in regularly cycling but infertile women in an attempt to understand possible causes of their infertility is, in the light of these findings, a physiologically justifiable procedure.