The effects of Cl⁻ and NO3- pretreatment on ³⁶Cl⁻influx and ClO3-/NO3- influx into Chara corallina cells were examined. Both treatments reduced ³⁶Cl⁻ influx into C. corallina cells in the acid pH range (4.5–7.0). NO3- pretreatment stimulated /NO3- influx into C. corallina cells, but Cl⁻ pretreatment had no effect. There was a direct inhibitory effect of CI on ClO3-/NO3- influx into C. corallina cells, but no apparent effect on net NONO3- uptake.The time course of ³⁶Cl accumulation into cytoplasmic and vacuolar compartments during incubation of the cells with ClO3- showed that significant radioactivity appeared in the vacuolar sap after 30 min. There was a linear increase in the percentage of total ³⁶CI which crossed the tonoplast (φ_c-v). There was no effect of Cl⁻ or NO3- pretreatment on accumulation of radioactivity in the vacuole. Thin layer chromatography of the vacuolar sap showed that after 2 h only one component was present which had an R_F which was similar to ³⁶CI⁻. The rate of accumulation of ³⁶C1⁻ in the vacuole could be used to estimate rates of ClO3- reduction.