The influence of Actinomycin D (AMD) applied intrahippocampally at doses of 1-6 mug/animal, on the acquisition and retention of a shock-motivated brightness discrimination was studied on rats in a semiautomatic Y-maze. The injection of AMD 4 hr prior to training did not influence the acquisition, but causes, dose-dependent, a retention loss in relearning 24 hr after training. Twenty-eight hr after AMD application, naive rats exhibited a deterioration of acquisition performance increasing equally with the dose. At the same time, both circumscribed necroses in the hippocampus and signs of a general intoxication were observed. Considering the described pro- and retroactive effects, it is concluded that the use of the inhibitor AMD in learning experiments is not suitable to provide reliable evidence of the specific importance of the cerebral RNA synthesis for memory consolidation.