In rats anaesthetized with urethane, push pull cannulae were stereotaxically introduced in the hippocampus (bilaterally) and Zn2+ assayed in the perfusate by atomic absorption spectrophotometry. When the cannula was located in the immediate vicinity of the mossy fiber zone, both spontaneous and K+ evoked release of Zn2+ were observed, this release was associated with a reduction in the histologically demonstrable Zn2+ as assessed by means of the Timm's stain. Neither spontaneous nor evoked release of Zn2+ was observed when the cannula was located in the medial part of CA1, the fimbria or the thalamus. These observations suggest that Zn2+ is released in the mossy fiber zone.