Polyamines are thought to play a role in the control of inherent or environmentally-induced growth rates of plants. To test this contention, we grew plants of four grass species, the inherently fast-growing Poa annua L. and Poa trivialis L. and the inherently slow-growing Poa compressa L. and Poa pratensis (L.) Schreb., at three levels of nitrate supply. Firstly, plants were compared when grown with free access to nitrate, allowing the plants to grow at their maximum relative growth rate (RGR(max)). Secondly, we compared the plants when grown with relative nitrate addition rates of 100 and 50 mmol N (mol N)-1 day-1 (RAR100 and RAR50, respectively). The freely-occurring polyamines, spermine, spermidine and putrescine, were separated from their conjugates; the latter were further subdivided into a TCA-soluble and a TCA-insoluble fraction. Each of the three fractions responded differently to the nitrate supply. Under nitrogen limitation, the total concentration of polyamines (free and bound ones together) decreased in both leaves and roots of all Poa species, whereas that in the stem remained more or less the same. These effects were to a large extent determined by the free polyamines. For the conjugates there was more differentiation, both between plant organ and among polyamine structures. A positive correlation between the RGR, LAR (leaf area per plant mass), SLA (leaf area per leaf mass), LMR (leaf mass per plant mass) and SMR (stem mass per plant mass) with the polyamine concentration was found. The RMR (root mass per plant mass) showed a negative one. No significant differences were found between the inherently fast- and slow-growing grass species. The (putrescine)/(spermine + spermidine) ratio in the leaves increased with decreasing nitrate supply, which is associated with a decrease in leaf expansion, accounting for a decrease in LAR and SLA. For the roots, this ratio tended to decrease with decreasing nitrate supply, whereas for the stems the results were somewhat more variable. We found no evidence for a crucial role of polyamines in the determination of inherent variation of growth in spite of a positive correlation of especially the free polyamines with growth parameters.