From any kind of living organisms, very weak spontaneous light emission without excitation light (ultraweak biochemiluminescence) is observed. We succeeded to detect ultraweak biochemiluminescence from rat hippocampal slice preparation at the order of 10(-19) W/mm2 by a single-photon detector using a silicon avalanche photodiode. It was shown that depolarization induced by the high concentration of potassium caused an increase in the intensity of biochemiluminescence from the rat hippocampal slice, and that suppression of neural activity by tetrodotoxin elicited a decrease of its luminescence. These findings suggest that correlation between the intensity of ultraweak biochemiluminescence and neural metabolic activity.