The toxicology of the cyanobacterial alkaloid cylindrospermopsin (CYN), a potent inhibitor of protein synthesis, appears complex and is not well understood. In exposed mice the liver is the main target for the toxic effects of CYN. In this study primary mouse hepatocyte cultures were used to investigate the mechanisms involved in CYN toxicity. The results show that 1–5 μM CYN caused significant concentration-dependent cytotoxicity (52%–82% cell death) at 18 h. Protein synthesis inhibition was a sensitive, early indicator of cellular responses to CYN. Following removal of the toxin, the inhibition of protein synthesis could not be reversed, showing behavior similar to that of the irreversible inhibitor emetine. In contrast to the LDH leakage, protein synthesis was maximally inhibited by 0.5 μM CYN. No protein synthesis occurred over 4–18 h at or above this concentration. Inhibition of cytochrome P450 (CYP450) activity with 50 μM proadifen or 50 μM ketoconazole diminished the toxicity of CYN but not the effects on protein synthesis. These findings imply a dissociation of the two events and implicate the involvement of CYP450-derived metabolites in the toxicity process, but not in the impairment of protein synthesis. Thus, the total abolition of protein synthesis may exaggerate the metabolite effects but cannot be considered a primary cause of cell death in hepatocytes over an acute time frame. In cell types deficient in CYP450 enzymes, protein synthesis inhibition may play a more crucial role in the development of cytotoxicity. © 2003 Wiley Periodicals, Inc. Environ Toxicol 18: 243–251, 2003.