Domoic acid is a shellfish toxin which produces gastrointestinal distress, followed by neurological symptoms such as headache, confusion, disorientation and severe deficits in short-term memory. Domoic acid is an amino acid which contains three carboxylic groups, and one imino group, and its solubility, rate of absorption, and elimination would vary depending on the protonation of these groups at different pH's. We propose that domoic acid toxicity varies with pH of administered domoic acid solution. Domoic acid toxicity was measured in mice as the onset times for scratching behaviour, seizure activity, and death, after the intraperitoneal administration of domoic acid at different pH's. Results of the present study show that the scratching behaviour, seizure activity, and death, occurred at 12, 40, and 55 min, after intraperitoneal administration of domoic acid at pH 3.7. Apparently, the onset times for three types of behaviours were relatively long, and well separated from each other. Domoic acid toxicity was lowest at pH 3.7, and highest at pH 7.4, with intermediate toxicity at other pH's. The onset time of scratching behaviour was not influenced by pH of domoic acid solution at three different doses. In contrast, the onset times for seizure activity, and death were significantly affected by pH of domoic acid, toxicity being higher at pH 7.4 than at pH 3.7. The pH effect on domoic acid toxicity diminished as the dose of domoic acid was increased. In fact, at 14.5 mg/kg domoic acid toxicity was similar at both pH's of 3.7 and 7.4. It is concluded that in vivo toxicity of domoic acid varies depending on pH of the administered solution. The differential toxicity of domoic acid at different pH may be related to its solubility, rate of absorption, and elimination, depending on the degree of protonation of domoic acid molecule. Domoic acid toxicity would also vary depending on the age of animal, receptor sensitivity and density in different regions of brain.