Porphyra ?erforata J. Ag. was collected from a rocky landfill site near Kitsilano Beach, Vancouver, British Columbia, Canada and was grown for 4 d in media with one of the following forms of inorganic nitrogen: NO;-, NH + and NO;-plus NH + and for 10 d in nitrogen-free media. Internal nitrogen accumulation (nitrate, ammonium, amino acids and soluble protein), nitrate and ammonium uptake rates, and nitrate reductase activity were measured daily. Short initial periods (10 to 20 min) of rapid ammonium uptake were common in nitrogen-deficient plants. In the case of nitrate uptake, initial uptake rates were low, increasing after 10 to 20 rain. Ammonium inhibited nitrate uptake for only the first 10 to 20 rain and then nitrate uptake rates were independent of ammonium concentration. Nitrogen starvation for 8 d overcame this initial suppression of nitrate uptake by ammonium. Nitrogen starvation also resulted in a decrease in soluble internal nitrate content and a transient increase in nitrate reductase activity. Little or no decrease was observed in internal ammonium, total amino acids and soluble protein. The cultures grown on nitrate only, maintained high ammonium uptake rates also. The rate of nitrate reduction may have limited the supply of nitrogen available for further assimilation. Internal nitrate concentrations were inversely correlated with nitrate uptake rates. Except for ammoniumgrown cultures, internal total amino acids and soluble protein showed no correlation with uptake rates. Both internal pool concentrations and enzyme activities are required to interpret changes in uptake rate during growth.