The reproducible steady-state carbon dioxide (CO2) photoassimilation potentials of three mutants and a normal form of pea (Pisum sativum L.) have been compared. The three mutants studied differed markedly in foliar morphology: genotype af af Tl Tl had leaflets converted to tendrils; Af Af tl tl had tendrils converted to leaflets; af af tl tl had relatively minute leaflets on a branched petiole. Interest lay primarily in the phenotype with only tendrils since it provided a potential means of reducing the volume of haulm that has to be rapidly processed in the case of vining peas, and dried in the case of harvest peas. These mutants had been derived from relatively unimproved cultivars. Before completion of the lengthy backcrossing required to make a full assessment of the value of such mutants an interim study using infra red gas analysis indicated that, in terms of CO2 photoassimilation per unit area of youngest expanded attached leaf of glasshouse-grown plants, the mutants were comparable to normal. The phenotype with only tendrils was the least efficient of those assayed at utilizing light of an intensity below 100 J m2 sec1 (400–700 nm), and on a unit dry-weight basis it was only 18 per cent as efficient as a normal-leaved pea. The other mutants were comparable to normal in this respect. Comparison of CO2 photoassimilation of glasshouse-grown and field-grown plants showed them to be similar though they differed in dry weight, transpiration, and dark respiration.