Neither acquisition learning nor 24-h retention was significantly altered by 6-hydroxydopamine intracerebral injections which depleted forebrain noradrenaline (NA) to less than 5% of control values. The absence of passive avoidance impairment cannot be ascribed to functional recovery following the lesion (indicated by testing 24 h post-operation) and by using the F-344 strain of rat which does not show denervation supersensitivity as measured by NA-sensitive adenylate cyclase. Nonassociative freezing to electric footshock, changed by the injections, resulted in slower acquisition at a footshock level 4 mA, but not at 1 mA.