1. The purpose of this study was to observe the effects of metyrapone on the time of oviposition and LH-stimulated steroidogenesis by granulosa cells and small yellow follicles. 2. In experiment 1, White Leghorn hens were injected for 11 d with 240 mg metyrapone 5 h before 'lights off'. Control hens were injected with 1 ml of vehicle (PEG-400). Metyrapone treatment resulted in a 28% decrease in the rate of lay and the modal frequency of the time of oviposition was phase-shifted by 15 h. 3. In experiment 2, hens were injected with 240 mg metyrapone 5 h before 'lights off' or at 'lights on'. While metyrapone treatment reduced the rate of lay, a clear phase-shift in the distribution of oviposition was not observed. Basal and LH-stimulated progesterone synthesis by the granulosa cells of the largest follicle and oestradiol synthesis by small yellow follicles was significantly reduced. 4. Metyrapone treatment significantly reduced basal, but not LH-stimulated output of androstenedione by whole small yellow follicles compared to that observed in control hens. 5. The addition of metyrapone in vitro to isolated granulosa cells from the three largest preovulatory follicles inhibited LH-stimulated progesterone production in a dose-specific manner. 6. The results of this study suggest that the ability of metyrapone to perturb the open-period is a pharmacological effect mediated through inhibition of ovarian and adrenal steroidogenesis.