Research on the effect of the benzodiazepines, Valium, and Librium on human psychomotor and cognitive functions is reviewed. Benzodiazepines which are the most important antianxiety medications also have anticonvulsant, hypnotic-sedative, and muscle-relaxant properties. Research on the benzodiazepine hypnotic "hangover" effects on cognitive and motor behavior is cited. The benzodiazepines Valium and Librium probably interact with neurotransmitters, especially GABA and very likely have specific receptors in the brain and central nervous system. Absorption and elimination rate vary with dosage, method of administration, and age. Valium and Librium have no gravely harmful side effects, little addictive potential; danger from overdosage is minimal. Although controlled studies of the impact of psychoactive drugs on psychomotor and cognitive performance are relatively recent, Valium and Librium apparently have little, if any, adverse effect on well established higher mental functions and may affect the speed with which simple repetitive motor actions are performed. None of their effects are irreversible. Benzodiazepines (BZ) have been remarkable drugs. They have virtually replaced all other forms of antianxiety medications (48, 95, 109, 225). All the BZ drugs additionally have anticonvulsant, sedative-hypnotic, and muscle-relaxant properties (4, 77, 88, 112, 252). Two of the BZ drugs, Valium (diazepam) and Librium (chlordiazepoxide) have been the best sellers of the BZ drug family and the most frequently prescribed drugs in the world (7, 15, 17, 77, 110, 137, 215, 257). The impact of Valium and Librium on human psychomotor and cognitive functions is the focus of this review of research. Since millions of people are using these drugs, how do Valium and Librium affect alertness and responsiveness, for example, in driving a car to work, or operating a machine in a factory (240)? Tranquilizing drugs like Valium and Librium were hailed when they replaced sedatives like barbiturates because they did not cloud the mind. Is decision-making or mental alertness affected in those who use Valium or Librium (69)? In studying the impact of drugs on the central nervous system (CNS) and brain, animal subjects frequently are employed. However, the human condition of anxiety for which Valium and Librium are usually prescribed is hard to evaluate and human subjects vary greatly, so that this review of research has been limited for the most part to studies with human subjects (8, 26, 50, 107, 108, 262, 263, 264).