Examination of 65 rectal biopsies and 10 resections by the histochemical detection of cholinesterase leads to the following conclusions: 1. Density of the nerve fibers in lamina propria is irregular and the examination of the cholinesterase activity may lead to falsely negative results in aganglionosis. 2. Constant is, in aganglionosis, the finding of markedly increased reaction in the nerve fibers of the muscularis mucosae and submucosa. 3. Biopsy must, therefore, include even muscularis mucosae and the choice of the instrument for biopsy removal must meet this requirement.