The binding study on nicotinoids, neonicotinoids and the related compounds using [3H]a-bungarotoxin and [3H]nicotine as probes revealed that binding to the recognition site of nicotinic acetylcholine receptor (nAChR) in insects requires the molecules to have 3-pyridylmethylamino moiety, while high ionization in vertebrates. When [3H]phencyclidine was used as a probe for the ion channel opening of Torpedo nAChR, it was indicated that high ionization of the molecule is important to be agonistic, although the presence of 3-pyridylmethylamino moiety is preferred. 15N NMR of the amino nitrogen atom of nicotinoids and the corresponding one of neonicotinoids indicated that the latter was far deshielded as compared with the former. The result implies that the unshared electron pair on the concerned nitrogen atom is delocalized by the presence of strong electron-withdrawing group and the nitrogen atom becomes partially positive. Such nitrogen is enough to interact with the insect nAChR, but not with the vertebrate one. The overall results explain why nicotine is highly toxic to mammals and rather limited in insecticidal activity, whereas imidacloprid is highly insecticidal and low in mammalian toxicity.