Various experiments involving the measurement of new, regenerated and total productivity using ¹⁵N and ¹³C tracers were carried out in the Bay of Bengal (BOB) and in the Arabian Sea. Results from ¹⁵N tracer experiments indicate that nitrate uptake can be underestimated by experiments with incubation time > 4 hours. Indirect evidence suggests pico- and nano-phytoplankton, on their dominance over microphytoplankton, can also influence the f-ratios. Difference in energy requirement for assimilation of different nitrogen compounds decides the preferred nitrogen source during the early hours of incubation. Variation in light intensity during incubation also plays a significant role in the assimilation of nitrogen. Results from time course experiments with both ¹⁵N and ¹³C tracers suggest that photoinhibition appears significant in BOB and the Arabian Sea during noon. A significant correlation has been found in the productivity values obtained using ¹⁵N and ¹³C tracers.