The effects of μ agonists fentanyl, methadone and morphine and K agonist U50,488 on behavior maintained by negative reinforcement were determined. Rats were trained on concurrent schedules in which pressing one lever postponed shock on a Sidman avoidance schedule and pressing the other lever produced signaled periods of timeout from avoidance on variable-ratio schedules. All of the μ agonists decreased responding maintained by timeout from avoidance at doses that increased or did not affect avoidance rates. The K agonist U50,488 decreased response rates in some rats, but increased responding in others. In no case was a selective reduction in responding on the timeout lever produced by U50,488. Thus, the previously reported selective decreases in timeout responding by morphine are also produced by μ agonists fentanyl and methadone, but not by K agonist U50,488.