Behavioral Assays Number of defeats, latency to first defeat, freezing behavior, and exploratory locomotion were scored from the recorded social defeat sessions. Freezing behavior was recorded whenever the intruder remained motionless for at least 2 sec, and is reported as percent time during the direct interaction between the intruder and the resident. Exploratory locomotion was recorded every time the intruder crossed with all four paws from one third of the cage into the adj acent third of the cage. Vertical lines were drawn on the video image of each resident' s home cage and exploratory locomotion was scored from the videotapes. Exploratory locomotion is reported as lines crossed per minute of direct interaction time. Two trained observers scored the social defeat stress videos from Experiments la and lb and inter-observer reliability was assessed. Immobility was recorded during the Porsolt swim tests whenever the rat balanced on its tail, completely motionless, or exhibited only slight forepaw movement for a minimum of 2 sec. This behavior is reported as total time spent immobile during the swim test. Two trained observers scored the swim test videos from Experiment 2 and inter-observer reliability was assessed. Histological Assays Immediately after decapitation in experiments la and lb, 6 mL of trunk blood from each rat was collected in polypropylene tubes on ice with 600 Cl1 of Na2EDTA at 20 Cpg/CLL. The blood samples were then immediately centrifuged at 1000 ref for 5 min and the plasma fraction frozen in 300 Cl1 aliquots at -800C. The brain from each intruder was quickly removed, frozen in 2-methylbutane at -400C, and stored at -800C for future analysis of molecular variables involved in HPA axis function. The thymus and adrenal glands were removed from each rat and frozen separately at -800C. These glands were