The activities and molecular forms of cholinesterases were studied in a collection of primary brain tumors consisting of primarily gliomas and meningiomas, together with samples of forebrain taken postmortem from patients suffering from diseases unrelated to the nervous system. Both types of tumors, as well as normal forebrain, contained substantial amounts of cholinesterase activity and some gliomas contained exceptionally high levels. In both normal forebrain and meningiomas, acetylcholinesterase (acetylcholine hydrolase; EC 3.1.1.7) accounted for almost all the cholinesterase activity, but in almost all gliomas elevated pseudocholinesterase (acylcholine acylhydrolase; EC 3.1.1.8) could be detected. The cholinesterase activity of both normal forebrain and gliomas migrated on sucrose gradients as a major component of 10-11 S together with a minor component of 4-5 S. In meningiomas a light (4.5 S) form was the principal component.