A paired pulse stimulation protocol was employed to examine the loss of inhibition in the hippocampus of the kainic acid-treated rat in vivo. Extracellular recordings were obtained from the CA1 pyramidal cell layer 1, 4, 8 and 16 weeks after a unilateral intracerebroventricular injection of kainic acid. Recordings were made both ipsilateral and contralateral to the site of kainic acid injection. The results demonstrated a loss of paired pulse inhibition in hippocampal CA1 area ipsilateral to the site of kainic acid injection which did not alter over 16 weeks. While the contralateral hippocampus showed no change 1 week after kainic acid injection, a reduction of paired pulse inhibition was seen after 4, 8 and 16 weeks. It is therefore apparent that the effects of kainic acid on the state of inhibition of the hippocampus are long-lasting and, furthermore, that the response to an injection of kainic acid is widespread, affecting the inhibitory control of the contralateral hippocampus.