IN Great Britain the most common poisons at the present time are the 5 : 5′ disubstituted barbiturates. The longer-acting barbiturates, barbitone and phenobarbitone, can accumulate in the body when administered in relatively low therapeutic dosage, and concentrations of these drugs in the blood of about 5 mgm./100 ml. do not necessarily imply poisoning. The shorter-acting barbiturates, when taken in therapeutic quantities, however, do not accumulate, and in such cases this concentration would imply that an excessive quantity had been taken.