The effects of recombinant luteinizing hormone (rLH) and recombinant follicle stimulating hormone (rFSH) both separately and together were analysed on the in-vitro maturation of a well-defined class of mouse early preantral follicles. Metaphase-II (MII) oocytes were only obtained when rLH or rFSH or a combination of both hormones was added to a standard rich culture medium at constant doses throughout the culture period. Without gonadotrophins only 8% of the follicles reached day 12 of culture. Follicle survival was optimal (between 90 and 100%) when rFSH was used; significantly fewer follicles were kept intact when rLH was added as the only supplement (54%; P < 0.001). Theca cells played a fundamental role in follicle survival when rLH was the only supplement. Presence of rLH significantly influenced the formation of antral-like cavities: on day 12, 59% of follicles cultured with rLH + rFSH had a large cavity, as compared with only 21% in cultures with rFSH only (P < 0.0001). The rHCG stimulated germinal vesicle breakdown rate from the different gonadotrophin-supplemented cultures on day 12 was not significantly different (FSH: 89%; LH: 70%; FSH/LH: 83%). However, addition of rLH to rFSH did significantly improve (P < 0.05) the completion of the first meiotic division up to the MII stage (FSH: 46%; LH: 39%; FSH/LH: 76%). Interfollicle differences in steroid production were very large and depended on the presence of theca cells and on the kind of gonadotrophin supplemented. Without theca cells, oestrogen concentrations in rFSH-supplemented cultures were 100 times lower than their theca-containing counterparts during the entire culture period. From culture day 9 onwards most conditioned media had increased basal progesterone concentrations. This study points up the in-vitro role of LH on the differentiation of follicle-like structures and the oocyte's meiotic maturation and suggests an important role for theca cells in providing modulators of in-vitro gonadotrophin action.