We found NADPH-diaphorase (presumably identical with nitric oxide synthase) in pyramidal neurones of the hippocampus in slices that stayed in a chamber for 30 min longer. In some instances parallel slices showed normal membrane properties when studied electrophysiologically. In freshly made slices the pyramidal neurones were not stained. Thus, after induction of the enzyme, the hippocampal pyramidal neurones can synthesize nitric oxide which may serve as a retrograde messenger in long-term potentiation. The enzyme may also play a role in cell loss seen in slices which stayed in a chamber for 9–22 h before fixation.