We investigated whether the large phenotypic variation in pyrrolizidine alkaloid (PA) concentration in Senecio jacobaea has a genetic component. A diallel cross and a half-sib analysis showed that 50-100% of the total variation in total PA concentration is due to genetic differences in the laboratory. For a large number of the major individual PAs significant heritabilities (for concentration as well as composition) were found in a nutrient-stressed and drought-stressed environment in half-sib analyses. PA concentrations differed significantly between the nutrient and drought treatments. © 1993.