The present paper deals with effects of some drugs affecting brain serotonergic mechanism on behaviour in cockroach (Periplaneta americana) and immature form (larvae) of ant lion (Myrmeleon formicarius). The effects of para-chlorophenylalanine, LSD-25 and 5-HTP on simple learning mechanisms in cockroach and on behaviour of ant lion (building the funnel-shape traps) were studied. Both p-chlorophenylalanine and LSD-25 facilitated behaviour of insects whilst 5-HTP delayed responses. Our experiments indicate that serotonin may play an inhibitory role upon behavioural processes in some invertebrates.