Oxytocin (OT) administration has been shown to inhibit adrenocorticotropic hormone (ACTH)&slash;cortisol secretion in several experimental conditions. In the present study, the plasma OT responses to suckling in 7 lactating women or to mechanical breast stimulation in 6 normally menstruating women (experimental tests) or to sham stimuli in the same subjects (control tests) were measured and correlated with the simultaneous changes in plasma ACTH/cortisol levels. All women showed similar basal levels of OT, ACTH and cortisol, which remained unmodified after sham stimulation. In contrast, both suckling and breast stimulation produced a significant increase in plasma OT levels and a significant decrease in plasma ACTH concentrations. When OT and ACTH data were considered together, a significant negative correlation was found between the OT increase and the simultaneous ACTH decline. Plasma cortisol levels were lower during suckling or breast stimulation than in control conditions. These data show an inverse relationship between plasma OT and ACTH levels during suckling and breast stimulation in humans, suggesting an inhibitory influence of OT on ACTH/cortisol secretion in a physiological condition.