In the present study, we investigated whether and how chronic morphine administration changes seizure susceptibility in rats. The role of morphine-dependence on the seizure susceptibility has been evaluated with models of pentylenetetrazol (PTZ)-kindling and acute convulsions induced by PTZ, N-methyl-D-aspartic acid (NMDA), picrotoxin and caffeine in adult male rats. The results showed that morphine-dependence increased seizure severity only at 1-4th PTZ injections in the kindling model. In acute convulsion tests, dependent rats demonstrated a significantly lower seizure threshold only for PTZ, while they demonstrated a significantly lower tendency to show tonic-clonic convulsions only for NMDA. It is concluded that morphine-dependence may modulate PTZ-kindling and seizure susceptibility in rats with emphasis on the role of GABA and NMDA neurotransmitter systems.