Observations made during the years 1968 through 1970 on the spatial and temporal variations in temperature, salinity, dissolved oxygen and pH in Pulicat Lake are presented. Water temperature showed double oscillation in its seasonal changes, whereas salinity had a single peak during May/June. From the mouth to the upper reaches of the lake there was a decreasing salinity gradient during monsoon and post-monsoon, which got more or less reversed during pre-monsoon and summer. Higher values of dissolved oxygen were observed during monsoon and post-monsoon when compared to summer and pre-monsoon. 'Hydrogen ion concentration showed an inverse relationship with oxygen. Wider fluctuations in the environmental parameters were observed in the northern sector of the lake and the probable reasons for this are discussed. An attempt has been made to correlate the variations in salinity with the fish production of the lake.