Abstract: By means of a quantitative HPTLC scanning method, changes in 7 active constituents of the toot of "Mudan" (Paeonia suffruticosa) and "Shaoyao" (P. lactiflora) were determined. The specimens were collected in different ontogenetic stages. The 7 constituents were paeoniflorin (Ⅰ), benzoylpaeoniflorin (Ⅱ), oxypaeoniflorin (Ⅲ), paeonol (Ⅳ), paeonoside (Ⅴ), apiopaeonoside (Ⅵ) and paeonolide (Ⅶ).The results showed that all these constituents were higher from May to June and September to October, and lower in April and from July to August respectively. The best collecting time should be in early and late spring or in autumn.