Ibotenic acid and muscimol are considered the principal components responsible of the hallucinogenic properties of Amanita muscaria. In this paper a sensitive ion-interaction HPLC method is presented which permits the simultaneous determination of ibotenic acid and muscimol. A C18 reverse phase is the stationary phase and the mobile phase is an aqueous solution of 5.0 mM octylammonium o-phosphate. A spectrophotometric comparative detection performed at 230 and 254 nm is employed. Detection limits are of 18 µg/L for ibotenic acid and of 30 µg/L for muscimol. Different sample pretreatment processes were compared. The highest recovery yields together with the minimum matrix effect were obtained by a procedure which only consists in the sonication of the fresh sample and its squeezing. The extract obtained is then diluted as required and filtered (0.2 µm) before injection. This preparation offers the further advantage to maximally preserve the native composition. The method was applied in the analysis of Amanita muscaria and permitted to evaluate the content of muscimol and ibotenic acid in heads and stems of the mushrooms. The same analysis performed for samples of Amanita citrina confirmed the absence in this species of both ibotenic acid and muscimol.