Saxitoxins have been found in Australian populations of the cyanobacterium Anabaena circinalis. The C-toxins (C1 and C2) and gonyautoxins (GTX2 and GTX3) are dominant components, while saxitoxin (STX), GTX5 and decarbamoyl gonyautoxins (dcGTX2 and dcGTX3) are minor constituents. Variation in concentration and composition of saxitoxins has been observed in natural populations and cultured strains and may reflect environmental conditions. Laboratory experiments were conducted with a single strain of A. circinalis to examine the effect of different nitrogen sources (dissolved atmospheric nitrogen, nitrate or ammonium) and varying concentrations of nitrate (0.0028, 0.28 and 28 mg N L^-1) on cyanobacterial growth and levels of saxitoxins. Growth was determined by cell enumeration and concentrations of saxitoxins were analysed by HPLC. All experiments indicated a consistent linear relationship between cell density and concentration of saxitoxins (intracellular + extracellular). Growth of A. circinalis was depressed by addition of ammonium (0.04 mg N L^-1) and by high levels of nitrate (28 mg N L^-1) and these treatments were associated with an increased toxin release. Concentration of extracellular saxitoxins increased with the age of cultures. The composition of intracellular and extracellular toxin profiles were usually similar, however the relative abundance of the different toxins was not always comparable. Extracellular toxin profiles generally comprised a higher proportion of STX and GTX2 and less C1. There was a strong correlation between toxin quota (concentration of saxitoxins per cell) and logarithmic growth rate in three of four experiments.