Environmental concentrations of CYN were monitored by Bogialli et al. (2006) in the eutrophic Albano Lake in Italy, during a 4 month period, using liquid chromatography and tandem mass spectrometry. The authors reported variations in total CYN content (intra and extracellular) between 0.41 and 18.4 mg/L in the contaminated waters. Variations were related with the month and the depth of sampling. Highest CYN concentrations were verified in the surface waters (up to 5 m depth). More recently, Gallo et al. (2009) detected extracellular CYN in the Averno Lake, south Italy, at 0.58 and 1.8 mg/L. These data indicate that 2.5 mg/L of CYN, considered in this work, can be present in contaminated superficial waters being environmentally relevant. The effects verified in the rice plants emphasize the potential consequences of the use of such waters, in plant growth and physiology. The impact to the environment, food production and animal and human health attributed to the water use can be increased if we consider that CYN is a relatively stable compound (Chiswell et al., 1999), persisting in the water after cyanobacterial senescence (Eaglesham et al., 1999).