Domoic acid, a structural analogue of kainic acid, has been identified as the toxin that poisoned people who consumed contaminated blue mussels harvested from eastern Prince Edward Island in December of 1987. To investigate the pharmacology of domoic acid in vivo we injected groups of mice with serial dilutions of extracts of contaminated mussels and verified domoic acid concentrations using high performance liquid chromatography. Mice progressed through a series of behavioural changes that were both reproducible and dose-dependent. These behaviours formed the basis of a rating scale that was used to reliably quantitate domoic acid concentrations as low as 20 micrograms/mL. This scale was then used to compare the relative toxicity of domoic acid contained in four formulations: namely, (1) extracts of contaminated mussels, (2) pure domoic acid, (3) extracts of noncontaminated mussels that were "spiked" with pure domoate, and (4) extracts of the algal source of domoic acid. Interpolation of the resulting dose-response curves produced median toxic dose (TD50) values of 2.9, 3.9, 4.9, and 4.2 mg/kg for the four formulations, respectively. Statistical analysis of these data revealed that curves for all formulations of domoic acid were parallel, but that extracts of contaminated mussels were significantly more potent than any of the other formulations at low and intermediate doses of domoic acid. We further compared domoic acid toxicity with that produced by kainic acid. Dose-response curves for both compounds were statistically parallel and both toxins were equally efficacious. The TD50 values were 3.9 and 31.9 mg/kg for pure domoic acid and kainic acid, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)