To investigate whether uptake of dissolved organic compounds might favour the growth potential of toxic cyanobacteria and dinoflagellates, surface water was collected in late July 1997 in the Gulf of Riga, and enriched with different nitrogen sources. (1) Organic substances were added (25 mu M N) under P-, Si- and Fe-replete conditions. Urea sustained a biomass increase (protein content) 20% greater than that with NH sub(4) super(+). Glycine, hypoxanthine and putrescine gave only 50% of the biomass obtained with NH sub(4) super(+). Glutamic acid and spermine supported growth in only one case each, and guanine in none. (2) The >1000 dalton fraction of dissolved organic matter (DOM) taken from the Daugava River was provided at concentrations of 5, 15 and 49 mu M dissolved organic nitrogen (DON); P, Si and Fe were not added, except in the control. DON sustained algal growth as much as ammonium and NO sub(3) super(-), but data were somewhat inconsistent. (3) The cyanobacterium Microcystis aeruginosa was cultured in filtered (0.45 mu m) water supplemented with organic substances (25 mu M N) and DOM (15 mu M DON), under P-, Si-, and Fe-replete conditions. Growth with urea, glycine, hypoxanthine, putrescine or spermine ranged between 145 and 175% of that obtained with NO sub(3) super(-). That 15 mu M DON sustained a biomass increase representing only 70% that gained with NO sub(3) super(-) might be explained by the different relative concentrations of DON and NO sub(3) super(-). Altogether, it is inferred that urea is an important potential nitrogen source for summer phytoplankton as a whole, while dissolved free amino acids (DFAA) and DON of terrestrial origin may partly sustain growth in some individual species.