Forty leghorn chickens at the commencement of egg laying, were divided into two groups, each with 10 females and 10 males. One group was exposed to the odour of synthetic pyrazine (2-methoxy-3-isobutylpyrazine) for 16 weeks while the other acted as a control. During the first 4 weeks the hens exposed to pyrazine odour laid significantly fewer eggs (unfertilized) than the controls, but thereafter both groups laid a similar number. Mean egg mass of the pyrazine exposed hens was significantly (P=0.012) greater (5.5%) than that of the control group. No significant difference was found in body mass or eggshell thickness. Similarly, there were no consistent significant differences between groups in oestrogen and testosterone concentrations in the blood of females and males, respectively. No pyrazine could be detected in cloacal extracts. The experiment shows that an external odour can affect the internal reproductive system of the chicken. It is suggested that the pyrazine-engendered increase in egg mass involves neuroendocrine regulation within the hypothalamus rather than hormonal interactions 'downstream' of the brain.