Homologous low molecular weight compounds, blocking postsynaptic glutamate receptors, were isolated from the venom of the spider Argiope lobata by ion exchange chromatography and reverse phase HPLC. Structures of nine blocking agents were determined by 1H-NMR and mass spectroscopy. Two-dimensional COSY spectra were used to identify spin systems of the protons coupled via two or three chemical bonds. The spin systems were combined into the sequence by means of the nuclear Overhauser effect between spatially close protons. Ionogenic groups were identified from pH dependences of the proton chemical shifts. Molecular structures thus obtained were checked against amino acid analysis and mass spectrometry data. The antagonists can be divided into three groups: argiopin, argiopinins and pseudoargiopinins depending on features of the chromophoric moiety (2,4-dihydroxyphenylacetic acid, 4-hydroxy-indole-3-acetic acid or indole-3-acetic acid).