Abstract The interaction of the neurotoxic natural products, kainic and ibotenic acids, both of which are also excitatory neurotransmitters and amino acid analogues of glutamic acid, along with the latter compound, with human erythrocyte membranes has been investigated by electron spin resonance methods. Only ibotenic acid caused a statistically significant alteration in the physical state of membrane proteins (P = 0.01) while none of these excitotoxins measurably affected motion of membrane lipids. In order to further investigate some of the molecular characteristics of ibotenic acid that may have contributed to its effect on the conformation of membrane proteins, similar spin labeling studies were performed employing the decarboxylation product and parent ring compound of this excitotoxin, muscimol and isoxazole, respectively. No effect of either of these latter compounds was observed suggesting that the carboxylic acid group of ibotenic acid is essential for its interaction with membrane proteins. These results are discussed in relation to the known different neurotoxic and physiological effects of kainic and ibotenic acids and muscimol.