Pretrial injections of pentylenetetrazole had two major effects on discrimination learning: they slowed the speed of response, whether running down an alley or bar pressing in a Skinner box, and at the same time enhanced the ability to learn both successive and simultaneous discriminations. In simultaneous discrimination situations the effects of pretrial injections of methylphenoxypropane were essentially opposite to those of pentylenetetrazole. The effects of pentylenetetrazole appear to be dose-dependent, and it is suggested that it might act by broadening the range of stimulus situation cues to which the animal will respond at any one time.