Tolerance to gamma-hydroxybutyrate (GHB) has been suggested in illicit users and has been described for the hypnotic effect in the rat. The aim of this study was to investigate whether tolerance is also observed for the EEG effect, and whether the EEG can give insight into the pharmacodynamic aspects of GHB tolerance. In three series of experiments, rats were pre-treated with either the GHB precursor gamma-butyrolactone (GBL) or saline intraperitoneally twice daily. In the first series, a reduction in sleeping time was observed in the GBL pre-treated rats compared with controls. In the second series, a fast infusion of GHB (300 mg kg(-1) over 5 min) was given after 10 days pre-treatment. The GHB plasma concentration-time curves showed a slightly faster decrease in GHB concentration in the GBL pre-treated rats, suggesting a small induction of the GHB metabolism (V(max) = 2882 +/- 457 microg min(-1) kg(-1) vs 2205 +/- 315 microg min(-1) kg(-1), P < 0.01). In contrast to controls, GBL pre-treated rats did not lose righting reflex. In the third series, a slow infusion of 480 mg kg(-1) h(-1) was given after 7 days pre-treatment, which allowed fitting a sigmoid E(max) model to the EEG amplitude versus GHB plasma concentration curve. This showed reduced end-organ sensitivity to GHB in the GBL pre-treated rats (EC50 (concentration required to obtain 50% depression of the baseline effect) = 653+/- 183 microg mL(-1) vs 323 +/- 68 microg mL(-1), P < 0.001). In conclusion, chronic pre-treatment with gamma-butyrolactone in the rat results in a reduced sleeping time and this tolerance is reflected by the EEG. This can mainly be explained by reduced end-organ sensitivity.