The neuropsychological effects of the GABA-reuptake blocker, tiagabine-HCl, were tested in an open trial of 22 adult patients with refractory partial epilepsy followed by a double-blind, placebo-controlled, cross-over trial in 12 subjects. Nineteen patients completed the initial open titration and fixed-dose phase of the study and 11 patients completed the double-blind phase. The median daily tiagabine dose was 32 mg during the open fixed dose and 24 mg during the double-blind periods. Neuropsychological evaluation did not show any significant effect on cognitive function in the open or double-blind phases. In this group of patients no statistically significant difference in the frequency of the total number of seizures or complex partial seizures was found in the open or double-blind stages. Seizure severity was significantly less in the open fixed dose than in the baseline period, but was not significantly different between the two double-blind periods. Reported side effects were transient, most commonly aggression/irritability, lethargy, headache and drowsiness. No significant EEG changes were observed.