A series of experiments with natural phytoplccnkton populations was made to study the photosynthetic Pmax and a indices, under different phases of the diel cycles. By "in situ" and simulated experiments the exogenous and endogenous influences on the photosynthesis and total chlorophyll-a were studied. The results obtained indicate the diel variation can introduce a great deal of error in the determination of the primary production. The use of constant values for a and Pmax in the equation for the determination of primary production without carrying for the aspect above as well as for the seasonal variation may give results for the diurnal values, with a very lend accuracy and without ecological significance. It was also observed that the measurements of chlorophyll-a varied within the relatively short period of time, required to perform a determination of photosynthesis. It is suggested that the Pmax, a and Ik values used in primary productivity methodology should be revised, as the indices considered are based in chlorophyll concentration.