Lower shock intensity: impairment or enhancement of retention by naloxone. Step-through latency (mean ± SEM) in seconds on the test trial for the No Swim-Vehicle group (n = 9), No Swim-Nal group (n = 9), Swim-Vehicle group (n = 8), and Swim-Nal group (n = 8). With shock intensity set at 0.45 mA (0.5 s) during passive-avoidance training, naloxone impaired retention when administered after swim-stress and enhanced retention when administered in the absence of swim-stress. *p < .05; **p < .005 compared with the corresponding vehicle group. p-values shown are for significant protected-t tests following one-way ANOVA; F(3, 30) = 5.90, p < .01.