gamma-Hydroxybutyric acid (GHB) produces a constellation of EEG and behavioral changes when given to animals, which represent an experimental model of generalized absence seizures. gamma-Butyrolactone (GBL), the prodrug of GHB, produces these changes more rapidly and consistently than GHB, such that the rat treated with GBL is a more reproducible and predictable model of absence seizures. The hypothesis that the epileptogenic effects of GBL are due solely to its conversion to GHB was tested. The regional concentration of both compounds in brain was determined in time-course and dose-response studies, as well as at the onset of EEG changes, induced by both GHB and GBL. Also, the EEG and behavioral effects of both drugs were ascertained after intrathalamic injection in the rat. gamma-Butyrolactone produced a rapid onset of bilaterally synchronous spike and wave discharges in the rat, which correlated with a rapid appearance of GHB in brain in the GBL-treated animals. In the GHB-treated animals, EEG changes occurred 20 min after administration of GHB when levels of GHB in the brain were peaking. The threshold concentration of GHB in brain for EEG changes, in both GHB-and GBL-treated animals was 240 x 10(-6) M. The concentration of GBL in brain peaked 1 min after administration of GBL and fell rapidly to undetectable levels within 5 min. Bilateral injection of GHB into thalamus resulted in a brief burst of spike and wave discharges, while GBL, administered into the thalamus, had no effect. The use of GBL as a prodrug for GHB in this model of generalized absence seizures is valid, since GBL itself was inactive.