Pharmacological exploration of the central nervous system carried out by means of a series of tests confirms the affinity of 3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylic acid alpha-cyano-3-phenoxybenzyl ester (delta-methrin) for the nervous system and reveals the following activities of deltamethrin: Deltamethrin has no neuroleptic or sedative action. It inhibits motility, sense of balance and inquisitiveness of treated animals. It potentiates chloral-induced hypnotic action but not that of pentobarbital. Not only does it have no antagonistic action on convulsivant agents: electric shock, pentetrazol, strychnine, but it stimulates their toxicity and prolongs the convulsive seizures. The fact that deltamethrin acts on different convulsivant agents (i.e. pentetrazol and strychnine) with various sites of action suggests that its activity is located in both the cortical and the medullary centres. Deltamethrin reveals the latent toxicity of tryptamine used in the non-toxic dose range; this predicts that deltamethrin might have an IMAO (inhibitor of monoamine oxidase) activity. Experiments carried out under the conditions adopted showed that deltamethrin acts on various sites.