Simple and rapid methods have been developed for the extraction and simultaneous determination of the three active diterpenoids, andrographolide (AP1), 14-deoxy-11,12-didehydroandrographolide (AP3) and neoandrographolide (AP4), from various samples of Andrographis paniculata (Burm.f.) Nees. Methanol extracts from the dried leaves, stems and crude products were analysed by isocratic HPLC using a methanol and water mobile phase with monitoring at 220 nm. There was a large variation of the three active diterpenoids in different A. paniculata products obtained from Thai markets. The results indicated that the amounts of these active compounds consumed, based on the recommended daily doses, from materials obtained from the different suppliers will be different. In addition, the stability of these three active compounds was also examined in dry herbs stored at room temperature. The results showed that andrographolide was more stable than the others. In contrast, the content of 14-deoxy-11,12-didehydroandrographolide increased and the neoandrographolide content fluctuated during storage time. The combination of different levels of these compounds in the source materials and the changes during storage could have a significant effect on the efficacy of this traditional herbal medicine in clinical treatment.