Eight subjects came individually to the sleep laboratory for two consecutive nights. On both nights they learnt a word list and attempted verbal free recall immediately and again after a 20 min. delay. They were instructed to verbalize their entire thought content (as far as possible) throughout list presentation. In a balanced design, learning was preceded by wakefulness or 30 min. sleep. Delayed memory and percentage forgetting were found to be impaired after prior sleep, as were rehearsal and recoding. A measure of the intensity of rehearsal and recoding was found to be related to delayed recall and forgetting but not to immediate recall. It was suggested that the memory loss after sleep was at least in part due to a failure of rehearsal and recoding at the time of learning.