Faecal measurements of oestradiol and testosterone allow the non-invasive estimation of plasma steroid concentrations in the domestic fowl.

1. Radioimmunoassays were validated for oestradiol and testosterone in faecal and plasma samples from domestic fowls. Faecal samples were prepared in phosphate buffer. A delay of up to 24 h between defaecation and the freezing of a dropping did not affect faecal steroid concentrations. 2. Differences in plasma concentrations of testosterone and oestradiol between cockerels and hens were clearly reflected in faecal hormone concentrations. Faecal concentrations of oestradiol were low in males and did not overlap with much higher concentrations in females, whereas there was some overlap between the sexes in faecal concentrations of testosterone. 3. The correlation coefficients (r2) between plasma and faecal steroid concentrations were 0.464 for testosterone and 0.852 for oestradiol (log-transformed data). The coefficients did not increase when the mean hormone concentrations for several droppings produced by each bird during a collection period were used. 4. Faecal steroid concentrations can be used as a measure of plasma steroid concentrations and hence of gonadal activity in chickens. The resolution of the faecal steroid method is less than if plasma measurements were used, but it offers a practical alternative to blood sampling that is non-invasive and does not require birds to be handled.