Changes in the defence compounds pyrrolizidine alkaloids (PAs) in roots and shoots of Senecio jacobaea are reported in response to nutrient addition in order to investigate whether changes in concentration are adaptive. PA concentrations were examined in leaves and roots of 40 vegetative ragwort plants, subjected to four nutrient treatments in a climate chamber study. Roots from 10 plants were subdivided into main root cortex, main root vascular cylinder, lateral roots and root tips and analysed for PA concentrations. Increasing nutrients lead to a significant reduction in total PA concentration of both roots and shoots. All individual PAs except jacobine decreased in concentration. The total amount of PA produced in the whole plant was not influenced by nutrient supply. Root tips contained a three times lower concentration than the main and lateral roots. The concentrations in the main root cortex were five times higher than concentrations in the vascular cylinder. Changes in biomass rather than changes in production rates can explain alterations in PA concentration of S. jacobaea in response to nutrients.