The present experiment was run to test the hypothesis that, when shock was signaled, rats would develop effective coping responses so as to reduce the current flow through them. A l-sec shock was delivered through a grid floor by a fixed impedance ac shock source. The current-flow measure was taken over the last 30 of 90 trials given over 3 days and indexed by "gross skin conductance" or GSC (shock). The rat under the signaled shock condition (n= 15) showed higher GSC (shock) than did the rats under the unsignaled shock condition (n=14). Thus, the result contradicted the hypothesis. There was no indication that the rats developed any preparatory response during the 5-sec signal, in terms of either GSC (signal] or posture. The results were discussed with reference to the preparatory-response hypothesis and various other possibilities.A considerable number of rat experiments conducted both in the present writers' laboratory (lmada & Okamura, 1975;Imada & Soga, 1971; Nageishi & Imada, 1974)and in other laboratories (e.g., Seligman, 1968;Seligman & Meyer, 1970) have shown that signaled electric shocks are less stressful than unsignaled shocks. In conditioned suppression experiments, rats' basal rate of licking or of leverpressing was shown to be suppressed less when brief electric shocks were given with a warning signal than when they were given unsignaled. It is also known that animals choose a situation in which shocks are signaled rather than unsignaled. This phenomenon is generally known as PSS (preference for signaled shock) and was first shown by Lockard (1963). Since, in all the above studies, the shocks rats received were made exactly equal, both in intensity and duration, regardless of condition, the demonstrated difference in stress in rats under the signaled and unsignaled conditions has been ascribed to the psychological effect of "unpredictability" of shock.Fanselow (1980) has recently summarized the major hypotheses that have been put forward to ac-. count for the above facts: the information hypothesis (Berlyne, 1960), the preparatory-response hypothesis (perkins, 1968), the safety-signal hypothesis (Seligman, 1968), the time-allocation hypothesis (Rachlin, 1976), and a new hypothesis based on Pavlovian condition-

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Effects of bilateral hippocampal lesions were investigated in a newly developed discriminated rearing-avoidance task. A significant facilitation of avoidance conditioning was found in rats with hippocampal lesions. The over-all escape topography of the hippocampal-lesioned rats was mostly jumping whereas that of the controls was rearing. These findings suggest that the hippocampal lesion increases reactivity to shock.

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The present experiments were conducted to obtain basic data on some factors that could influence rat's coping processes in grid-shock situations by measuring the current flow through the rat. Unsignaled 1-s shocks were delivered repeatedly through a grid floor by a fixed impedance ac shock source. The current-flow measure was indexed by " gross skin conductance (GSC)". In Experiment I, the shock intensity was the independent variable, and rat's GSCs under four shock levels (75 V, 150 V, 300 V and 600 V) were measured. The rats under the higher shock conditions showed higher GSC than did the rats under the lower shock conditions. In Experiment II, the effects of water deprivation (0 h, 24 h, 48 h and 72 h) upon rat's GSCs were investigated. Generally, the higher thirst has led to lower GSC. In Experiment II when the grid-floor rods were thicker and the space between rods wider than in Experiment I, there was less tendency for rats to jump and move about, which have led to higher GSC and smaller variability in GSCs than Experiment I.