We have examined the effects of a subanesthetic dose of ethanol on the entry into the brains of rats of three 14C-labelled substances which are known to affect brain function. All substances were given intraperitoneally. When ethanol was given the concentration of pentobarbital in the brain reached and was maintained at a higher level than without ethanol. The barbiturate entered the brain and became distributed between plasma, red cells, and brain extremely rapidly in the presence or absence of ethanol. Administration of ethanol decreased the rate of breakdown of pentobarbital, so that higher levels of unchanged pentobarbital were maintained in blood, liver, and brain. Breakdown of the barbiturate occurred rapidly in liver but almost no radioactive breakdown products entered or were formed in the brain. Thiamine entered the brain sparingly but, after 60 min, the ratio of radioactivity per gram of brain to that per milliliter of plasma became considerably greater with ethanol than without. In the absence or presence of ethanol this ratio increased with time due to a rapid disappearance of radioactivity from the plasma while radioactivity in the brain tended to increase slightly. In the presence of ethanol the rate of disappearance from the plasma increased, presumably due to increased uptake and metabolism of thiamine by other tissues. Urinary excretion of radioactivity decreased. Injected γ-aminobutyric acid (GABA), entered the brain very sparingly and much of what entered was metabolized. Ethanol had no effect on the endogenous GABA concentration in the brain nor on the entry or metabolism of injected GABA.