The content of cyanogenic glucosides was determined in seeds of Hevea brasiliensis ( cultivars FX 25 and IAN 873), H. pauciflora, H. benthamiana and H. camargoana collected in Brazil. Large amounts of the monoglucoside linamarin were detected in all seeds tested but lotaustralin, which occurred in much lower concentrations, did not occur in every individual seed. After storage (at 27o C and 50;RH) the diglucosides linustatin and neolinustatin were also detectable, implying that the 2 monoglycosides were glucosylated during aging. During germination and seedling development, the patterns of developmental changes in HCN-potential differed between the species and within the H. brasiliensis cultivars. In the wild type IAN 873 seedlings the HCN-potential fell to less than 15;of the initial content whereas it remained nearly constant in FX 25 seedlings. In the other Hevea species the potential first increased and then decreased. These changes depended on the conversion of cyanogenic glucosides to non-cyanogenic compounds and on de novo synthesis of cyanogens.