Gibberellin production by Gibberella fujikuroi started only after the nitrogen source was depleted and ceased upon its renewal. Nitrogen repression of gibberellin biosynthesis is not an indirect effect of the growth arrest that follows the depletion of an essential nutrient because gibberellins were not produced upon depletion of phosphate. Mycelia produced gibberellins when suspended in a glucose solution. Production ceased some time after depletion of glucose and resumed upon its readdition. Under certain conditions, the gibberellin production rate was inversely proportional to the glucose concentrations. The specific regulation of gibberellin biosynthesis by the nitrogen source imposes a revision of the concept that gibberellins are secondary metabolites whose production is triggered by imbalance or cessation of growth.