Horseradish peroxidase (HRP) has been widely used in neurobiology to trace neural pathways (axonal transport) and to correlate physiology with morphology (intracellular injection). We report that 5% HRP in 0.1 M phosphate buffered normal saline may be used to fill micropipettes for stable extracellular single cell recordings. HRP can then be iontophoresed at the desired recording site(s) and does not appear to impair activity of other neurons in the area or to cause damage to the electrode tip after the marking procedure. Subsequent perfusion, sectioning and reacting of the brain with the diamino-benzidine chromagen yielded a discrete (100-500 mu diameter) brown reaction product in the extracellular space, as well as peroxidase filled perikarya which were readily distinguishable from damaged vascular or neural elements. This method is highly reliable and provides a simple technique for localizing the tip of micropipette electrodes.