1. It has been shown that the glass electrode method as previously described by the author for measurements of transient rates in photosynthesis is free of systematic errors.
2. Justification has been given for the use of the steady state-gas flow method for measuring rates of assimilation under conditions in which the solution is known not to be seriously depleted of CO2.
3. The decline of photosynthetic rates under light saturation at CO2 pressures less than several tenths per cent of an atmosphere has been shown to be a real phenomenon. It has been suggested that there may be a real discrepancy between this finding and those of some other investigators due to a difference in the previous history of the algal suspensions.