The changes in AChE activity and protein content following cold or heat exposure and heat death were determined in the brain and spinal cord of both Rana ridibunda and Chalcides ocellatus. Cold exposure (10 degrees C) caused a decrease in the enzyme activity and protein content of both animals. Exposure to heat (36-40 degrees C) increased markedly the AChE activity and the amount of protein in the two experimental animals. Heat death was found to be associated with a prominent decrease in enzyme activity and the protein level of the brain and spinal cord of the two poikilotherms.