The acquisition and extinction of CRs in classical and instrumental avoidance eyelid conditioning were compared when the comparison was independent of the number and pattern of UCS occurrences. The effect of omitting vs. delayed onset of the UCS on some trials was also studied. The instrumental group showed a superior level of conditioning. The partial reinforcement procedure of UCS-omission resulted in greater resistance to extinction than UCS-delay. A classical conditioning group on 100% reinforcement was superior to all groups in acquisition but showed the least resistance to extinction. These and other findings are discussed in terms of several contemporary hypotheses of conditioning phenomena. (PsycINFO Database Record (c) 2006 APA, all rights reserved)