Plasticity of the acoustic startle reflex was measured in rats in which a complete transection between the forebrain and midbrain was made. During a period from 60 to 100 min after surgery, startle amplitude in the transected rats was relatively stable and comparable with that of the controls (which had been anesthetized with halothane and placed in a stereotaxic instrument). During this period the transection did not alter the temporal recovery process (with intervals of 2, 4, 8, or 16 sec) or auditory prepulse inhibition (with intervals of 25, 50, 100, 500, or 1,000 msec) or the normal reduction in startle caused by high levels of background noise. The transection did prevent the normal increase in startle caused by moderate levels of background noise and eliminated within-session habituation. The effect on habituation was particularly convincing since the curves of the transected and nontransected rats actually crossed. The results are discussed in terms of how the transection procedure can be used to evaluate various hypotheses about underlying mechanisms of startle plasticity.