Recently there has been developed a sensitive and specific method for the histochemical demonstration of certain monoamines, including noradrenaline. The original method involves treatment of the freeze-dried tissue with formaldehyde gas, which inactivates certain formalin-sensitive enzymes. A modification, using cryostat sections, was elaborated, which permits the demonstration of noradrenaline in adrenergic neurons, and allows the study of monoamines and for instance cholinesterases in adjacent sections of the same piece of tissue.