The effects of Ca2+ and ketamine on Ca2+/CaM PK II activity in rat hippocampal slices under in vitro hypoxic states have been studied and the effect of hypoxia on glutamate accumulation was also investigated. The results were as follows: (1) Ca2+/CaM PK II activity decreased gradually with increasing hypoxic time in culture medium with or without 1.3 Ca2- mmol/L, with the former case being much more pronounced. (2) Extracellular accumulation of GLU increased about two-fold after hypoxia for 30 min in vitro. (3) When slices were incubated for 30 min under conditions in excess of exogenous GLU the enzyme activity markedly decreased, a finding suggesting that the inhibition of enzyme activity induced by hypoxia may result from excitotoxicity. (4) The inhibition of the enzyme activity induced by either hypoxia or excess exogenous glutamate alone could be antagonized markedly by pretreatment with ketamine, suggesting that the hypoxia induced inhibition of the enzyme activity is mediated by NMDA receptor.