Nitrate uptake and nitrite efflux patterns in Nostoc MAC showed a rapid phase followed by their saturation. Nitrite efflux was maximum in nitrate medium whereas the cells incubated in N2 and NH 4 (+) media exhibited a decreased nitrite efflux activity. The simultaneous presence of NH 4 (+) and nitrate significantly decreased nitrite efflux. L-Methionine-DL-sulphoximine (MSX) prevented inhibition of nitrite efflux by NH 4 (+) . In the dark there was negligible nitrite efflux, whereas illumination increased the rate of nitrite efflux significantly. The nitrite efflux system was maximally operative at pH 8.0, 30ยฐC and a photon fluence rate of 50 ฮผmol m(-2). s(-1). These results confirm that (i) the nitrite efflux system in Nostoc MAC is dependent upon nitrate uptake and assimilation and is repressible by NH 4 (+) ; (ii) NH 4 (+) itself is not the actual repressor of nitrite efflux; a product of NH 4 (+) assimilation via glutamine synthetase (GS) is required for repression to occur; (iii) the catalytic function of GS does not appear to be involved in nitrate assimilation-dependent nitrite efflux, and (iv) the optimum pH, temperature and illumination for maximum nitrite efflux were found to be 8.0, 30ยฐC and 50ฮผmol m(-2). s respectively.