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Petuniolide C, an ergostane-type steroidal insecticide isolated from Petunia parodii, caused immediate uncoordinated activity and salivation in fourth instar nymphs of the desert locust, Schistocerca gregaria, when injected into the hemocoel at doses as low as 0.5 mu g insect(-1). The LD50 for petuniolide C via injection in locust nymphs (14.1 mu g insect(-1)) is nearly identical to that for the convulsant, picrotoxinin (14.0 mu g insect(-1)), a known inhibitor of the GABA (gamma-aminobutyric acid)-mediated chloride ionophore. Petuniolide C and picrotoxinin in combination had an additive effect on locust mortality. Following topical administration, petuniolide C caused knockdown and mortality in a cyclodiene-susceptible strain of the German cockroach, Blatella germanica, bur significantly reduced effects in a cyclodiene-resistant strain. Using an in vitro rat brain synaptic membrane preparation, both petuniolide C and a natural analog, petuniolide D, were shown to be weak Ligands of the GABA(a) receptor. (C) 1997 Academic Press.