The effects of iron deficiency on the growth and metabolism of isolated pea roots grown in sterile culture have been analysed. Changes were followed in whole routs sampled at intervals and in successive sections of roots sampled at a critical stage of deficiency. Transfer of deficient roots to complete nutrient after varying periods in the absence of iron showed the growth restriction was reversible for at least 6 days. Results suggest that the complete cessation of cell division after 6 days in iron-deficient culture was due primarily to a disturbance localized in the apical meristematic cells. Changes which occurred in the levels of protein nitrogen, ethanol-soluble nitrogen, amino acids, soluble carbohydrates and respiration are mainly attributed to the cessation of cell division rather than the cause of it. The rate of cell expansion was unaffected by iron deficiency but the final size of mature cells was reduced. Addition of iron to deficient roots stimulated cell division more rapidly than protein synthesis or respiration rate. Insofar as the serial-sectioning technique used enabled separation to be made of cell division and cell enlargement, the results suggest that the effect of iron deficiency is confined primarily to the former. In addition, the final phase of cell maturation may also be affected but the evidence was inconclusive and these effects may not be determined directly by depletion of the iron supply. Evidence is presented showing that the metabolic changes associated with the transition from cell division through cell expansion to cell maturation depend on the source of root inocula used. Seedling roots showed marked changes in metabolism and meristem size when first isolated due to the depletion of metabolites translocated from the germinated seed, and it was necessary to use inocula taken from cultured excised roots to provide valid data on the changes due to increasing cell age.