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Mice given ECS after training on an inhibitory avoidance task showed greater retrograde amnesia when training and treatments were given near the crest of the daily activity rhythm as measured by rectal temperature.Evidence has accumulated in recent years that fluctuations in activity levels along the 24-h continuum are characteristic of many organs and organ systems (Halberg, 1960;Halberg, Albrecht, & Bittner, 1959;Harner, 1961).Several studies have indicated that a number of biologic variables may be related to learning and memory and to the response to amnesic agents such as electroconvulsive shock (ECS). An interaction between strain and phase of the activity cycle has been shown to influence maze learning performance in two strains of inbred mice (Hostetter, 1966). Thresholds to electroshock convulsions in rats were observed to be significantly lower at night than in the early afternoon (Woolley & Timiras, 1962). Age (Tattan, 1957) and strain (Hudspeth, McGaugh, & Thomson, 1964; OliveriO, McGaugh, & Bovet, in preparation;Thompson & Dean, 1955) have been shown to influence the degree of amnesia produced by electroconvulsive shock.The preliminary experiments reported here were designed to investigate: (1) whether learning and memory in an inhibitory avoidance task in routine-standardized animals will vary with the rectal temperature rhythm, and (2) whether the amnesic effects of electroconvulsive shock following avoidance training will vary as a function of this rhythm. SubjectsSwiss Webster male mice approximately 60 days of age were used as Ss. They were housed four animals per cage with ad lib food and water. Cages were kept on open shelves in a small quiet room lighted by a fluorescent ceiling fixture from 4 a.m. to 4 p.m. and completely dark from 4 p.m. to 4 a.m. Room temperature was maintained at about 22 o C. Animals were taken from the colony room at least eight days prior to the experiments, and kept in this standardization room prior to and during the experimental period. The rectal temperature rhythm was spot checked with a Yellow Springs Telethermometer. In these animals, under these conditions, rectal temperature averaged 34.9±.69 0 C at 1 p.m. and 36.5 ± 1.7 o C at 9 p.m. ApparatusThe apparatus for inhibitory avoidance training was that described by McGaugh & Alpern (1966 GWEN STEPHENS, JAMES L. McGAUGH AND H. P. ALPERN UNIVERSITY OF CALIFORNIA, IRVINEtraining electroshock consisted of a current of apprOximately 8 rnA delivered for .8 sec by way of corneal electrodes. The animal was held in the E's hand during ECS administration. Resuscitation via the nose and mouth by means of a plastic bottle and tube was administered to all animals immediately after the shock.EXPERIMENT 1 Seventy-two animals were divided into SIX groups of 12 animals each. starting at 1 p.m. each animal of the first three groups was given three training trials in immediate succession on the inhibitory avoidance learning task. For each trial the mouse was placed on a small lighted metal platform in front of a hole leading into...

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