Recent studies have suggested a paracrine role for several fibroblast growth factors (FGFs) in the regulation of follicle and luteal development. Fgf13 is a non-secreted FGF that has been previously localized to the developing gonads, but it is not known if it is expressed in the adult ovary. The objective of the present study was to determine the expression pattern of Fgf13 mRNA in the bovine ovary. Fgf13 mRNA expression was examined by semiquantitative RT-PCR using Gapdh as the internal control gene in theca and granulosa cells, corpora lutea (CL) and oocytes collected from abattoir ovaries. Follicles were grouped according to estradiol content (<5, 5-20, >20-100 and >100 ng/ml) and size (5-7, 8-10 and >10 mm diameter). CL samples were morphologically classified into four developmental stages. Fgf13 mRNA expression was assessed in pools containing 50 oocytes aspirated from follicles larger than 4 mm in diameter. ANOVA was used to test for the main effects of follicle size group, and estradiol concentration group in granulosa and theca cells, and to test the effect of CL developmental stage on Fgf13 mRNA abundance. Fgf13 mRNA was detected in the CL and in somatic follicle cells, but not in oocytes. Thecal Fgf13 expression increased with increasing follicle diameter but did not change with intrafollicular estradiol concentrations. No evidence of developmental regulation of Fgf13 mRNA expression was observed in granulosa cells and CL. The present data demonstrate for the first time the expression of an intracellular FGF in the bovine ovary and suggests that Fgf13 mRNA is upregulated in bovine theca cells during antral follicle growth.