Graafian follicles from mature New Zealand white rabbits were incubated for 4 hr at 37°C with air as the gas phase. The medium was changed every 15 min for 1 hr. Medium containing ovine LH was then added for four consecutive medium changes. Samples of the medium were analysed for 17 β-hydroxyandrogens, oestrogens and progestins by radioimmunoassay procedures. When no LH was present, progestin and 17 β-hydroxyandrogen secretion declined to negligible levels within 2 hr. Addition of 10 μg ovine LH to the medium for 1 hr caused an increase in 17 β-hydroxyandrogen secretion which continued for the duration of the experiment. No significant change was observed in oestrogen secretion but there was a slight increase in progestin secretion. Addition of 5 μg and 1 μg LH for 15 min also caused an elevation of 17 β-hydroxyandrogen secretion within 1 hr whereas progestin secretion took longer to increase and no change in oestrogen secretion was observed. No effect was observed with LH concentrations of less than 500 pg/ml for 1 hr, but 1 and 10 ng LH/ml also caused an increase in 17 β-hydroxyandrogen secretion. Oestrogen secretion was also stimulated at these levels. Control incubations with interstitial tissue showed no oestrogen secretion, slight secretion of 17 β-hydroxyandrogens and greater secretion of progestin which was stimulated by exogenous LH.