The effects of nalorphine were examined under a shock titration procedure in which squirrel monkeys received shock during 15-s period. If the monkey did not make five responses during the 15-s shock period, the shock was increased by one increment and maintained for another 15-s period. If five responses were made during the 15-s shock period, the shock was immediately terminated for 15s during which the chamber was dark (time out). After the 15-s time out period, the shock resumed at the next lower intensity. Three schedules of shock increment were examined in which shock increased from 0 to either 1.0, 2.0, or 5.5 mA. Under all three schedules of shock increment nalorphine decreased median shock levels and increased rates of responding in the presence of shock and in the absence of shock (i.e., during time out). These effects were apparent during hour 1 of the session. Responding during hours 2 and 3 of the session was unchanged.