The primary factor determining the observed decrease in active C1− influx during salt accumulation in carrot and barley root cells has been shown to be the concentration of C1− + NO3− in the vacuole. The relationship between C1− influx and the vacuolar concentrations of various substances was examined after the tissues had accumulated ions from various salt solutions. After accumulating K+ malate, C1− influx was not reduced, but after accumulating C1− or NO3− salts, C1− influx was reduced by up to 90 per cent. Considering all treatments, C1− influx was not correlated with the vacuolar concentration of K+, Na+, (K++Na+), reducing sugars, malate, C1−, or NO3−, nor with the cellular osmotic pressure. The correlation coefficient between Cl− influx and log (C1− + NO3− concentration in the vacuole) was highly significant, and accounted for all the variation in C1− influx in this experiment. Net NO3− influx is similarly reduced by a high C1− concentration in the vacuole. External Cl− and NO3− have quantitatively different, apparently competitive, effects on C1− influx. These differ from the apparently negative-feedback effects of C1− and NO3− in the vacuole, which are quantitatively similar. Decreasing the internal hydrostatic pressure by raising the external osmotic pressure increased active K+ influx in Valonia ventricosa, but had no effect on C1− or K+ influx in carrot or maize root cells. Cl− influx is not related to the reducing sugar concentration during ageing drifts in excised carrot root tissue. Acetazolamide did not inhibit C1− influx to carrot tissue. The implications of this type of negative feedback regulation, and the relationship between C1− and NO3− transport are discussed.