Methylglyoxal (10(-5) to 1.5 X 10(-4) M) was found to have excitatory effects on synaptic transmission in the isolated 6th abdominal ganglion of the cockroach. There was a concentration-dependent depolarization of the giant interneurones which was accompanied by an increase in the amplitude and duration of electrically evoked excitatory postsynaptic potentials. The frequency of spontaneous activity was also increased.