: In investigating mechanisms underlying intrauterine growth retardation (IUGR), circulating melatonin and cortisol were radioimmunoassayed. Blood samples were collected every 4 hours during 24 hours on a strict 24-hour standardized routine in hospital from two groups of women in their third trimester of pregnancy. One group consisted of 14 healthy, uncomplicated pregnancies (HAGA); the other group consisted of 11 pregnancies complicated by intrauterine growth retardation (IUGR) confirmed at birth. The circadian characteristics of melatonin and cortisol were assessed for each woman and compared between the two groups by analyses of variance for repeated measures and by parameter tests based on the cosinor. Since a circasemiannual (about half-yearly) component prominently characterizes body weight and length at birth of children with birth characteristics below usual norms, the circadian characteristics of melatonin and cortisol were also analyzed transversely (across women within each group). The 24-hour average and the 24-hour and 12-hour amplitudes of melatonin of women in the IUGR, but not in the HAGA group, were indeed found to be modulated by an about half-yearly component. This study confirms the circadian rhythmicity of melatonin in healthy pregnant women and extends the finding to pregnancies complicated by IUGR, uncovering about half-yearly changes in melatonin in women with IUGR, thereby extending results obtained in healthy non-pregnant women and men. These variations may reflect influences from geomagnetic disturbances also characterized by a prominent half-yearly pattern, to which the pineal has been shown to be sensitive.