The inhibins and activins are dimeric growth factors with important regulatory functions during development. In this study, changes in inhibin subunit messenger RNA (mRNA) levels were measured in the ovary during early postnatal development in the normal mouse and the hypogonadal (hpg) mouse, which lacks circulating gonadotropins. Levels of inhibin alpha-, beta A-, and beta B-subunit mRNAs were measured relative to beta-actin using a semiquantitative reverse transcription-polymerase chain reaction technique. Transcripts encoding all three subunits were present at birth; the alpha-subunit was the most abundant, followed by beta A-subunit (6% of alpha) and beta B-subunit (0.4% of alpha). After birth, levels of all three subunit transcripts increased between 6- and 10-fold. Changes in inhibin beta A- and beta B-subunit levels were most marked around 7 days, the period of secondary follicle development, whereas alpha-subunit transcript levels increased constantly after day 1 to reach a peak at 10 days, when mature secondary follicles are present. In hpg mice, levels of ovarian inhibin alpha-subunit mRNA levels were normal at all ages up to 15 days. In contrast, inhibin beta A-subunit mRNA levels were normal at birth in hpg mice, but did not increase after that up to day 15. Levels of beta B-subunit mRNA were significantly lower than normal on day 1 in hpg mice and also failed to show a significant increase up to 15 days. These results show that inhibin subunit mRNA levels are differentially regulated during ovarian development in the mouse. Normal expression of beta-subunits is completely gonadotropin dependent around the period of late primary to secondary follicle development. The inhibin alpha-subunit, in contrast, is expressed at a high level during development independent of gonadotropin stimulation.