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If the paired-associate verbal learning situation is considered an analogue of the classical conditioning situation, then the presentation of the response member of the S-R pairs corresponds to the presentation of the UCS and reinforcement. Therefore variation in percentage occurrence of response members in the paired-associate situation should be analogous to variation in the percentage of reinforcement in the conditioning situation.At the time the present study was begun, percentage occurrence of response members (hereafter abbreviated ORM) had not been investigated in paired-associate verbal learning. Recently, Goss, Morgan, and Golin (1959) have reported that the mean number of trials to learn paired nonsense-syllable lists to criterion decreased as percentage ORM increased. Adjective lists were used in the present experiment to investigate the ORM dimension at five points; namely, 20%, 40%, 60%, 80%, and 100% ORM.It is well established that intermittently reinforced responses are typically more resistant to extinction than continuously reinforced responses. If an unlearning (Melton & Irwin, 1940;Melton & von Lackum, 1941) extinction-like (Underwood, 1948) process underlies forgetting, at least in part, then it is conceivable that a paired-associate list learned with in-

1 1 gratefully acknowledge the generous assistance and counsel given me by Dr. Benton J. Underwood and Dr. Carl P. Duncan in carrying out this research and in the preparation of the manuscript for publication. Dr. Quinn Mc-Nemar's advice concerning the statistical treatment of the data is sincerely appreciated. I also wish to thank Mr. Jack Richardson for his many helpful suggestions.

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