Peristimulus time histograms of extracellularly recorded action potential discharges of cortical neurons in response to single shock and/or repetitive stimulation of 'specific' and 'non-specific' nuclei of the thalamus were studied after i.m. penicillin injection during a period corresponding to that of the development of spike and wave (SW) discharges of feline generalized penicillin epilepsy (FGPE). After i.m. penicillin cortical neurons displayed an enhancement of both the excitatory and 'inhibitory' phases of their responses to single shock stimulation of n. centralis medialis (NCM). This increase was even more pronounced for responses induced by repetitive stimulation of NCM at the frequencies inducing typical recruiting responses. These changes always preceded the appearance of SW discharges. Changes of the responses of cortical neurons to single shock and repetitive stimulation of 'specific' thalamic nuclei after penicillin were weak and inconsistent, although when observed were characterized by an enhancement of both excitatory and 'inhibitory' phases. The latter appeared not to decrease after i.m. penicillin. These data suggest that the appearance of SW discharges of FGPE is closely related to an increased responsiveness of cortical neurons to thalamocortical volleys arising from the so-called 'non-specific' nuclei. This facilitation of the recruiting process is accompanied by an increase of both excitatory and 'inhibitory' phases of the cortical neuronal responses induced by the volleys.