Electrophysiological changes occurring during the development of a kindled focus in the CA1 region of the rat hippocampus were studied in vivo. The most conspicuous changes of the field potentials (EPs) recorded from the stratum radiatum to stimulation of the Schaffer collaterals were the following: (1) a progressive decrease of the slope of the decaying phase of the EPs which was significantly different (P less than 0.02) from controls from sessions 8-12 onwards; (2) in the EPs recorded from the stratum pyramidale/oriens a population spike emerged from sessions 7-10 onwards; the ratio amplitude of the population spike/slope of the local EP increased progressively from session 8 onwards until a saturation level was reached; and (3) a progressive attenuation of paired-pulse depression; this decreased linearly with kindling session from session 1 up to session 11 (r = 0.97, P less than 0.01) and thereafter stabilized. These results are interpreted as due to a progressive imbalance between excitatory and inhibitory processes resulting in a decrease of inhibitory control in CA1 accompanied by a decrease in threshold of pyramidal neurons.