Sorghum seedlings lacking one of the phytochromes, phyB, have elongated internode, suggesting that they may have an alteration in gibberellin physiology. To test the possibility that phyB mutations affect seedling gibberellin perception and metabolism, the responsiveness of wild-type and phyB-1 seedlings to exogenous GA₃ was investigated. The phyB-1 showed higher internode elongation rate than the wild type in response to lower concentrations of exogenous GA₃ application, showing that the mutation causes an increase in responsiveness to GA. However, at the higher concentrations of GA₃ application, phyB-1 and wild-type showed similar elongation rate, impling that responsiveness to higher concentrations of GA is not controlled by phytochrome. These results suggest that, although GAs are required for internode elongation, phyB does not act primarily by changing absolute levels of GAs but rather by decreasing seedling responsiveness to GAs at lower concentrations.