The effects of four stereoisomers of β-hydroxyglutamic acid (BHGA) and related substances were examined on the electrical activity of the PON (periodically oscillating neurone) identified in the subesophageal ganglia of the African giant snail (Achatina fulica Ferussac). 1) Erythro-L-(eL-) and threo-L-BHGA showed an inhibitory effect on the electrical activity of the PON, and critical concentrations of these two substances to produce the effect (by the bath application) were 10(-6)~3×10(-6) and 3×10(-5)~10(-4)kg/l respectively. Erythro-D- and threo-D-BHGA had no effect at 10(-4)kg/l. 2) Of the BHGA related substances examined, erythro-ibotenic acid (I acid), quisqualic acid (Q acid), three stereoisomers of tricholomic acid, α-methyl-DL-glutamic acid and erythro-DL-β-hydroxyaspartic acid showed an inhibitory effect on the PON, and the critical concentrations of these substances were respectively 3×10(-6)~10(-5),10(-5),10(-4),10(-4),10(-4),10(-4) and 10(-4)kg/l. 3) It was confirmed by the microdrop application that eL-BHGA, I acid and Q acid directly hyperpolarized the PON membrane. 4) In the chloride free medium, eL-BHGA, I acid and Q acid showed the same hyperpolarizing effect as in physiological medium by microdrop application. The hyperpolarization caused by these substances was not dependent on chloride ions. 5) The resistance of the PON membrane was not changed by eL-BHGA at 10(-5)kg/l as compared with the physiological state. But, under a higher concentration (10(-4)kg/l), it decreased. 6) eL-BHGA was considered as the inhibitory transmitter of the PON or as an agonist of an unknown inhibitory transmitters of the PON.