Abstract
1. The sensitivity of the central nervous system to barbiturate was determined in rats during the chronic administration of barbitone sodium and after its withdrawal.2. The brain barbiturate concentration determined on awakening from a hypnotic dose administered intraperitoneally was found to increase throughout the period of barbitone administration.3. A similar gradual development of central nervous system tolerance was indicated by measuring the duration of anaesthesia following an intraventricular injection of pentobarbitone.4. The change in sensitivity of the brain which occurred during the period of barbitone administration was not demonstrable from the measurement of sleeping time following intraperitoneal injection of barbitone or pentobarbitone.5. After withdrawal, the sensitivity of the brain to barbiturate gradually returned to normal.6. It was concluded that the hypersensitivity to pentobarbitone, but not to barbitone, which develops after withdrawal of barbitone sodium is due to a decreased drug-metabolizing capacity.