Petunia hybrida and Citrus paradisi have significantly different flavonoid accumulation patterns. Petunia sp. tend to accumulate flavonol glycosides and anthocyanins while Citrus paradisi is known for its accumulation of flavanone diglycosides. One possible point of regulation of flavanone metabolism is flavanone 3-hydroxylase (F3H) expression. To test whether this is a key factor in the different flavanone usage by Petunia hybrida and Citrus paradisi, F3H mRNA expression in seedlings of different developmental stages was measured using semi-quantitative RT-PCR. Primers were designed to conserved regions of F3H and used to amplify an approximately 350 bp segment for quantitation by PhosphorImaging. Primary leaves of 32 day old grapefruit seedlings and a grapefruit flower bud had the highest levels of F3H mRNA expression. Petunia seedlings had much lower levels of F3H mRNA expression relative to grapefruit. The highest expression in petunia was in primary leaves and roots of 65 day old seedlings. These results indicate that preferential use of naringenin for production of high levels of flavanone glycosides in young grapefruit leaves cannot be attributed to decreased F3H mRNA expression.