Modulation of plasma biotin in the fowl (Gallus domesticus) by oestrogen and ambient temperature.

1. Exposure of laying hens to elevated ambient temperatures (30-40 degrees C) resulted in a significant increase in the biotin concentrations in plasma and egg yolk. 2. Exogenous oestrogen administration to immature pullets increased plasma biotin three-fold. The increase was six-fold when the birds were simultaneously exposed to an ambient temperature of 35 degrees C. 3. It is proposed that ambient temperature affects the balance between thyroid and ovarian hormones resulting in increased circulating levels of biotin and deposition of the vitamin in egg yolk.