The cytochrome P450 aromatase (P450 _arom ) enzyme is required for bioconversion of androgen to oestrogen. In this study ovarian P450 _arom mRNA and enzyme activity have been measured during development in normal mice and hypogonadal ( hpg ) mice which lack circulating gonadotrophins. A semi-quantitative reverse transcription-PCR (RTPCR) technique was used to measure cytochrome P450 _arom mRNA levels and aromatase enzyme activity was measured directly. Using RT-PCR, P450 _arom mRNA was detectable in the adult mouse ovary and also in the uterus, kidney, brain and skeletal muscle but not in cardiac smooth muscle. In the normal mouse, P450 _arom mRNA was detectable in the ovary on the day of birth (day 1) and levels increased significantly up to day 15 with the most marked changes seen between days 1 and 5. Aromatase activity was also detectable at all ages in the ovary and increased significantly between days 1 and 7. In ovaries from [ill] mice, normal levels of P450 _arom mRNA were present on day 1 but there was no significant change in P450 _arom mRNA at later ages up to day 15. These results show that in the newborn mouse ovary, which contains only primordial follicles, there is a basal expression of P450 _arom mRNA which is not gonadotrophindependent. After 1 day, however, gonadotrophins are required for normal expression of ovarian P450 _arom and this coincides with development of primary and secondary follicles.