The amount of indole-3-acetic acid (IAA) was measured in peach fruits by gas chromatography-mass spectrometry-selective ion monitoring using an isotope dilution assay with [13C6]IAA as an internal standard throughout the growing season. Ethylene evolution of the fruit was also measured. IAA levels were 25 nanograms per gram fresh weight, 18 days after anthesis. Both UIA levels and rates of ethylene evolution declined to their lowest levels (7 nanograms IAA per gram fresh weight and 0.01 nanoliter ethylene per gram per hour) in the second stage of fruit growth. Endogenous levels of free-IAA and ethylene evolution increased in the last stage of peach fruit development to 32 nanograms per gram fresh weight and 0.27 nanoliter per gram per hour, respectively. IAA amounts peaked in the ovules 67 days after anthesis.