An investigation of the cortisol and prolactin responses accompanying acute melatonin suppression by light (600 lux) in humans is described. Light given from midnight to 0300h suppressed nocturnal plasma melatonin concentrations by 65%. Despite this significant suppression of melatonin, no significant effect on plasma cortisol or prolactin concentrations was observed. These data support recent studies which argue that, if there is a relationship between melatonin, the hypothalamo-pituitary, and the hypothalamo-pituitary-adrenal axis in humans, it is neither direct nor simple.