Objective To understand the relationship between isozyme activities and diterpene lactone biosynthesis of Andrographis paniculata. Methods Plants were collected during ontogeny from seeding to seed maturity, and separated into leaves and stems for determination. Morphological and yield parameters were used to describe plant growing states. Isozyme changes were tested by polyacrylamide gel electrophoresis. HPLC was used to develop the fingerprints as well as to determine the diterpene lactone content. Results Significant increases were observed in the activities of isozymes, such as aspartate aminotransferase(AST), malate dehydrogenase(MDH), peroxidase(POD), and catalase(CAT), around the early stage of bud in leaves, and the activities of these four kinds of isozymes increased gradually as time progressed in stems. The content changes of diterpene lactones in leaves and stem were various. In the leaves, andrographolide(1) was recorded the highest [(23.63 ± 1.06) mg/g] at the early stage of bud, whereas deoxyandrographolide(2) was the lowest [(6.78 ± 0.27) mg/g] at this period and it reached the highest level at the seeding stage [(26.05 ± 1.04) mg/g]. Dehydroandrographolide(3) and neoandrographolide(4) fluctuated during growing stages. Meanwhile, the HPLC fingerprint showed that the content changes of two unknown compounds were related to that of dehydroandrographolide in leaves. In stems, andrographolide had increased gradually until the bud stage [(8.26 ± 0.33)mg/g], and other three diterpene lactones showed a trend of fluctuation. The yield of total diterpene lactones in aerial part reached the highest at the first flowering stage(806.71 mg/plant). Conclusion These results lay the foundation for the future research on the relationship of isozymes and diterpene lactones, and for determining the most favorable time for harvesting A. paniculata. Objective To understand the relationship between isozyme activities and diterpene lactone biosynthesis of Andrographis paniculata. Methods Plants were collected during ontogeny from seeding to seed maturity, and separated into leaves and stems for determination. Morphological and yield parameters were used to describe plant growing states. Isozyme changes were tested by polyacrylamide gel electrophoresis. Results were found in the activities of isozymes, such as aspartate aminotransferase (AST), malate dehydrogenase (MDH), peroxidase (POD), and catalase (CAT), around the early stage of bud in leaves, and the activities of these four kinds of isozymes increased gradually as time progressed in stems. The content changes of diterpene lactones in leaves and stems various. In the leaves, andrographolide (1) was recorded the highest [(23.63 ± 1.06) mg / g] at the early stag The highest level at the seeding stage [(26.05 ± 1.04) mg / g]. Dehydroandrographolide (2) Also, the HPLC fingerprint showed that the content changes of two unknown compounds were related to that of dehydroandrographolide in leaves. In stems, andrographolide had increased gradually until the bud stage [(8.26 ± 0.33) The yield of total diterpene lactones in aerial part reached the highest at the first flowering stage (806.71 mg / plant). Conclusion These results lay the foundation for the future research on the relationship of isozymes and diterpene lactones, and for for the most favorable time for harvesting A. paniculata.