Antipsychotic agents which are considered to depressive to the central nervous system (CNS) but free of dependence liability were used in combination with barbiturates or tranquilizers to study the physical dependence liability in the so-enhanced CNS depression. In the study of physical dependence formation, when CNS depression was maintained in an enhanced stage during the continuous application of the drug combinations, the withdrawal convulsions were enhanced in both frequency and severity. In the crossphysical dependence study, two-drug combinations, i.e., phenobarbital (PhB)-chloropromazine (CPZ), PhB-diphenhydramine (DPH) and nitrazepam-chlorprothixene, and 3-drug combinations (i.e., PhB-CPZ-promethazine and PhB-CPZ-DPH were evaluated. The 2-drug combinations suppressed some of the withdrawal signs, but those at high dosages showed a tendency to aggravate the signs. The 3-drug combinations differed from the 2-drug combinations in that the suppression of withdrawal signs was synergistically enhanced and the barbital withdrawal signs were weak. In conclusion, when CNS depression with PhB was enhanced with combinations of dependence liability-free drugs the drug combinations enhanced withdrawal convulsions both in frequency and severity. The sedation enhanced by the combinations did not always parallel the suppression of barbital withdrawal.