The specific respiration rates of nodulated root systems, of nodules and of roots were determined during active nitrogen fixation in soya bean, navy bean, pea, lucerne, red clover and white clover, by measurements on whole plants before and after the removal of nodule populations. Similar measurements were made on comparable populations of the six legumes, lacking nodules but receiving abundant nitrate-nitrogen, to determine the specific respiration of their roots. All plants were grown in a controlled-environment climate which fostered rapid growth. The specific respiration rates of nodulated root systems of the three grain and three forage legumes during a 7–14-day period of vegetative growth varied between 10 and 17 mg CO2 g−1 (dry weight) h−1. This mean value consisted of two components: a specific root respiration rate of 6–9 mg CO2 g−1 h−1 and a specific nodule respiration rate of 22–46 mg CO2 g−1 h−1. Nodule respiration accounted for 42–70 per cent of nodulated root respiration; nodule weight accounted for 12–40 per cent of nodulated root weight. The specific respiration rates of roots lacking nodules and utilizing nitrate nitrogen were generally 20–30 per cent greater than the equivalent rates of roots from nodulated plants. The measured respiratory effluxes are discussed in the context of nitrogen nitrogen fixation, nitrate assimilation.