A recent study found that avoidance extinction is equally facilitated by response prevention (blocking) whether the latter involves CS-alone or CS-shock presentations. An experiment was performed to determine whether this result was due to the use of a lengthy shock (5 sec) during response prevention. Five groups of rats were extinguished: (1) without prior blocking, (2) after blocking with CS only, (3) after blocking with a lengthy (5 sec) CS-contingent shock, (4) after blocking with a brief (.5 sec) CS-contingent shock, or (5) after blocking with a brief (.5 sec) shock only. The group blocked with the brief CS-contingent shock was substantially more resistant to extinction than the other four groups. The unblocked group and the group blocked with brief shock only required more trials to extinguish than the groups blocked with CS only or with lengthy CS-contingent shock, but did not differ from each other. The groups blocked with CS only or with lengthy CS-contingent shock also failed to differ from one another. The data support a significant role for Pavlovian conditioning processes in the effect of response prevention upon avoidance extinction.