Application of a foot shock during the acquisition trial of a one-trial passive avoidance task is associated with a rise in the concentration of serotonin in the hippocampus 24 h after conclusion of the acquisition trial. Carbon dioxide (CO2) induces amnesia for the passive avoidance response when administered immediately upon termination of the acquisition trial. In rats subjected to CO2 treatment following foot shock the rise in hippocampal serotonin is not observed 24 h later. The vasopressin analogue desglycinamide lysine vasopressin attenuates CO2-induced amnesia for the passive avoidance response when given prior to either the acquisition or the retrieval test (24 h after acquisition). This attenuation of the passive avoidance response is associated with a rise in the hippocampal serotonin concentration similar to the one observed in non-amnesic animals. It is suggested that a correlation exists between changes in hippocampal serotonin metabolism and the retrievability of the passive avoidance response.