1. Variations in plasma concentrations of growth hormone (GH) and luteinising hormone (LH) were measured by radioimmunoassay in blood samples taken during the ovulatory cycle of the domestic hen.
2. A peak in LH concentration was observed 4 h prior to ovulation.
3. Plasma GH concentrations were quite variable but a significant increase was observed between 2 h before and 2 h after ovulation and the concentration remained elevated for about 4 h thereafter.