Variation over plant ontogeny can play an important role in shaping trade-offs between investing resources in growth, reproduction and defence. Most previous ontogenetic studies on plant defence have compared two life stages, making it impossible to follow complete ontogenetic trajectories for plant defence traits. 2. We used the highly defended rain forest understorey tree Ryparosa kurrangii to examine chemical (cyanogen concentrations; CNM) and physical (leaf mass per area; LMA) ontogenetic defence trajectories across multiple stages of development for the ?rst time, using ?eld and glass- house plants. 3. In glasshouse seedlings, total plant cyanide and CNM decreased between cotyledon-bearing and recently autonomous seedlings. In ?eld populations, foliar CNM decreased in a nonlinear fashion and showed a trade-off with LMA through the ontogenetic progression from small seedlings to large mature trees. 4. Cyanogenesis was ubiquitous in all individuals tested, with considerable quantitative plasticity in constitutive expression. Environmental in?uences on defence traits, as measured by foliar nitrogen and potential light availability in the ?eld, had no detectable effect on CNM. Non-cyanide nitrogen (N - NCN)M was negatively correlated with CNM, and NCN was negatively correlated with plant height; therefore, at constant plant height there was no relationship between (N - NCN)M and CNM. LMA was positively correlated with increasing light availability. 5. Our results suggest that light and nitrogen availability have no detectable effect on CNM in R. kurrangii and that most of the observed population-level CNM plasticity may be due to under- lying genetic and ontogenetic in?uences. These ?ndings are related to a theoretical model recently proposed for resistance trajectories during plant ontogeny, taking into account the life-history traits of rain forest understorey trees. 6. Synthesis. The nonlinear ontogenetic trajectory of plant defence expression observed in this study suggests that ontogenetic defence changes may be strongly in?uenced by plant life-history traits, the defence(s) examined and the environmental niche the plant occupies.