AbstractEvidence fromin vitrostudies strongly implicates basic fibroblast growth factor (bFGF) as a local regulator of ovarian function. However, thein vitrofunction of this growth factor in the ovary is uncertain. The objective of this study has thus been to investigate the biological role of bFGF in the rat ovary by monitoring bFGF gene expression usingin situhybridization in 3 systems; (1) the naturally cycling ovary, (2) ovaries of immature rats treated with diethylstilbesterol (DES), and (3) primary rat granulosa cell cultures. The rat estrus cycle can be divided into 4 stages as determined by vaginal cytology; diestrus, proestrus, estrus and metestrus. bFGF mRNA transcripts were localized to granulosa and theca cells of developing follicles during proestrus and estrus and in the corpus luteum following ovulation during metestrus. The estrogen analogue DES induced extensivein vitrofolliculogenesis and high levels of bFGF mRNA in both granulosa and theca cells when compared to controls. Detectable levels of bFGF mRNA were also observed in primary granulosa cell cultures grown to high density. Employment of thisin situhybridization procedure has enabled thein vitrocellular sources of bFGF mRNA to be identified and the time course of expression during the estrus cycle to be monitored. The biological significance of this expression and the interplay between bFGF, extra-and intra-ovarian modulators are discussed.