A method to determine ginkgolic acids in Ginkgo biloba leaves was developed on the basis of pyrolytic methylation-gas chromatography. Pyrolytic methylation of G. biloba leaves in the injector at 300 degrees C in the presence of an organic alkali, tetramethylammonium hydroxide (TMAH, 25% methanol), allowed quantitative analysis of ginkgolic acids as their corresponding methyl derivatives. On the basis of the relative areas of these peaks, chemical composition of the ginkgolic acids in G. biloba leaves was determined with the relative standard deviations (RSDs) less than 3.4% (n = 3). The calibration curves of 5 main ginkgolic acids showed good linearity with the correlation coefficients greater than 0.996 6. The detection limits were between 0.8 - 2.8 mg/kg for the 5 compounds (S/N = 3). It is a simple, accurate and highly sensitive method for the chemical analysis and quality control of ginkgolic acids in G. biloba leaves and extracts.