Cyanogenic plants release cyanide from endogenous cyanide-containing compounds (generally cyanogenic glycosides) and thus have an effective means of chemical defense. The capacity for cyanogenesis can be highly variable, even among individuals within a population. The genetic, environmental and developmental factors determining this variability are poorly understood, particularly in tree species. We used Eucalyptus polyanthemos Schauer subsp. vestita L. Johnson & K. Hill to quantify aspects of the regulation of cyanogenic capacity, which in this species is determined by foliar cyanogenic glycoside concentration. A half-sibling progeny trial, based on seed collected from open-pollinated trees covering a range of cyanogenic capacities, was used to assess the heritability of cyanogenesis in E. polyanthemos. Narrow sense heritability (h(2) +/- 1 SE) was estimated to be 0.82 +/- 0.20 from an intra-class correlation and 0.78 +/- 0.11 from a standardized progeny-parent regression. Foliar cyanogenic glycoside concentrations were on average about 70% lower in seedlings than in maternal trees, suggesting that there is a developmental delay in the accumulation of cyanogenic capacity in this species. The high h(2) values indicate that cyanogenic capacity is largely genetically determined and that environmental factors have little effect. To test this supposition, we grew seedlings at two soil nitrogen (N) concentrations (N influences cyanogenic capacity in some species) and found no appreciable effect on cyanogenic glycoside concentration, biomass partitioning or relative growth rate. Highly cyanogenic seedlings grew more slowly than seedlings with lower cyanogenic capacities, and relative growth rate was positively associated with net assimilation rate in seedlings in both N treatments.