Alterations in the enzymatic activity and antigenicity of Na,K-ATPase as well as changes in cochlear morphology were assessed in gerbil inner ears harvested at selected time intervals up to 18 h postmortem. Na,K-ATPase activity was assayed biochemically in one cochlea from each animal and the other cochlea was fixed and embedded in paraffin for evaluation by light microscopy. Na,K-ATPase antigenicity was assessed by immunostaining with a broad-spectrum antiserum reactive with all known isoforms of the enzyme, and structural preservation was evaluated on adjacent sections stained with hematoxylin and eosin. The results showed a downward trend in enzymatic activity of Na,K-ATPase in lateral wall tissues within 1 h of death. In contrast, Na,K-ATPase immunoreactivity was fairly well preserved with postmortem fixation delays up to 12 h, despite the considerable structural degradation of cochlear tissues which began 2-3 h postmortem. It is concluded that under controlled environmental conditions, cochleas collected up to 4 h postmortem are suitable for morphological and immunohistochemical study of Na,K-ATPase by light microscopy. Cochleas collected more than 5 h postmortem were useful only for relatively gross immunohistochemical studies. It is suggested that cochleas intended for biochemical assays of Na,K-ATPase and probably most other enzymes should be collected within 1 h of death.