The effect of the insecticide fipronil on non-target organisms was studied on rat brain slices and identified giant neurons of the pond snail Lymnaea stagnalis. This compound acts as an antagonist on GABA(A) receptors. Although fipronil has moderate mammalian toxicity, our experiments confirmed that it modifies neuronal excitability in the rat somatosensory cortex. The amplitudes of evoked field potentials increased significantly after 30 min fipronil treatment. Short-term plasticity was examined with paired-pulse stimulation, this phenomenon was not affected by fipronil. On the other hand, the efficacy of LTP-induction was enhanced in the treated slices. Fipronil is highly toxic to freshwater invertebrates, especially molluscs. In Lymnaea stagnalis, the firing pattern of a GABA receptor- containing neuron (RPeD1) was studied. On this neuron, GABA has an excitatory, hypopolarizing effect. Fipronil treatment decreased the action potential frequency in a concentration-dependent manner. On the membrane potential of the cell, it had a slightly hyperpolarizing effect. These experiments confirmed that fipronil toxicity is mediated by GABA receptors in the nervous system of invertebrates as well as vertebrates. These types of experiments may help in establishing tolerance levels of pesticide residues and in finding proper treatment in case of eventual poisonings.