We describe the development of a novel transformation system for pigeonpea (Cajanus cajan (L.) Millsp.) cell lines using particle bombardment. The aim of our study was not only to develop a transformation system for this important but recalcitrant species but to do so using a gene of interest and to study the behaviour of this transgene in a legume background. The gene we chose for our studies was the heterologous oat arginine decarboxylase cDNA. Transgenic lines engineered with this cDNA had a small, but significant increase in arginine decarboxylase activity (ADC; EC 4.1.1.17). We detected an increase in putrescine levels in all the lines we analysed irrespective of whether major or minor changes in ADC activity were observed. Spermidine and spermine levels were also increased in some of the lines. We confirmed that small changes in the adc message (mRNA) had a significant effect on enzyme activity and more importantly on polyamine levels. This is the first report describing modulation of a gene involved in a metabolic pathway in transgenic pigeonpea tissues.