Gibberellins (GAs) are a group of plant growth hormones that were first isolated from the fungus Gibberella fujikuroi. The biosynthesis of GA in liquid cultures of the fungus has been examined using high-performance liquid chromatography and combined gas chromatography-mass spectrometry. GA(3) was the predominant GA in well-aerated cultures. GA(4) and GA(7), intermediates in GA(3) biosynthesis, accumulated in cultures with low levels of dissolved oxygen, but were not detectable in more highly aerated cultures. Light stimulated the production of GA(3) in G. fujikuroi cultures grown from young stock cultures. Cell-free enzyme studies revealed a significant stimulation in the levels of kaurenoic acid oxidation in cultures grown in the light in comparison with those grown in the dark. However, measurements of the relative rates of [(14)C]mevalonic acid incorporation into kaurene showed no effect of light on this early part of the pathway. Preliminary experiments indicated that blue light is most effective in enhancing kaurenoic acid oxidation.