Apprehension over the adequncy of current techniques stimulated a detailed study of the time factor in the arsenate inhibition of growth and respiration in excised stem and root sections of Pisum sativum. Growth inhibition by arsenate sets in very slowly, its rate of onset being related to the molar concentration (C) of arsenate ate by the relation where T50 is the time taken in hours to reduce the growth rate to 50 per cent of the control and K is a constant. An explanation of the physiological basis of this relationship is attempted. Estimates were made of the final steady growth rate (relative to control) in various arsenate concentrations. The inhibitions calculated from this rate are held to approximate to the true arsenate effect and are shown to be very different from those calculated from ‘total growth’ measures. Respiration of growing stem sections is not inhibited by the low arsenate concentrations that inhibit growth. Some inhibition is indicated at high concentrations (3 × 10−4M. and over) but only after 15-20 hours of exposure. Two per cent sucrose has no effect on the arsenate inhibiition of stem growth. Sucrose, however, markedly stimulates respiration in stem sections, but this stimulation is prevented by arsenate. The misinterpretations which may arise as a result of ignoring the time factor in inhibition studies in excised organ sections are discussed and the desirability of constructing complete growth curves in all such studies is stressed.