The time course of respiratory parameters and blood pressure was studied in decerebrated rats (DR), decerebrated rats treated with a combination of thiopental plus urethane (DAR), and intact brain rats anesthetized with the same combination (IBAR). Moreover, the respiratory sensitivity to a stimulating dose of amphetamine was tested in the three preparations. DR exhibited a spontaneous and steady increase of ventilation which was absent in DAR. A steady increase of ventilation was also observed in IBAR, although of a lesser intensity. Amphetamine induced a clear respiratory stimulation which was decreased by the administration of anesthetics. A tendency to hypotension was seen in all animals. Therefore, the respiratory instability and the decreased pharmacological response observed in the presence of anesthetics are important factors to be considered when interpreting results obtained in this kind of preparations.