200 white rats were given 30 food-rewarded training trials on a horizontal-bar-pressing habit and 30 on a vertical-bar habit. Half of the animals were later allowed to perform the H-bar habit unrewarded until they reached a criterion of no response in a period of 5 minutes. Recovery of the H-bar habit was measured by a second extinction test made 5.5, 25, 65 or 185 minutes after the last response, a separate group of 25 animals being used for each interval. With the remaining 100 animals, following extinction of the V-bar habit, the recovery of the H-bar habit was measured (recovery from the generalized effects of extinction). Measured by number of responses and time to reach the 5-minute criterion, recovery from the specific effects of extinction was negatively accelerated, showing 50% and 75% recovery respectively in 185 minutes. Curves of recovery from generalized effects were approximately parallel to the curves of recovery from specific effects of extinction, but showed 85% and 100% recovery in terms of number of responses and time measures respectively. Measured by mean rate of response, there was no recovery in the ordinary meaning of the term.

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The apparatus has been described in detail by Hovland (7), with one exception: the response in the present experiment was recorded in millivolts by means of a Burr-Lane-Nims microvoltmeter (1) and a General Electric photo-electric recording milliammeter. The subject sat in a comfortable chair in a sound-shielded room. The tonal conditioned stimuli were delivered through earphones, and the shock through copper electrodes on the subject's left wrist. The response was obtained through polished silver (quarter-dollar) electrodes from the back and palm of the subject's right hand. The timing of stimulus presentation was controlled by a Dodge pendulum.

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