The two basins of Lake 302, in the Experimental Lakes Area, northwestern Ontario, were separated by means of vinyl barriers. Beginning in 1982 and continuing through 1984, H2SO4. was added to the south basin during the ice-free months. Alkalinity was reduced rapidly and pH was lowered from near 6.6 to 5.6 by 1984. Transmission of light increased, resulting in a deeper euphotic zone. Over the same time period, HNO3 was added to the north basin of Lake 302. Alkalinity was reduced much less in this basin and the pH dropped only to about 6.2 by 1984. Transmission of light did not increase. Mean epilimnetic phytoplankton production rates were greater in the HNO3 basin than in the H2SO4 basin. However, in the basin receiving H2SO4, epilimnetic rates remained at or above those in reference lakes, and mean production rates below the mixed layer were greater than in the HNO3 basin, probably because of increased light penetration. Mean euphotic zone production was similar in both basins and comparable to that measured in reference lakes. Phytoplankton community photosynthesis was not adversely affected by the additions of either HNO3 or H2SO4.