The respiratory effects of pymadin, amiridine, thyrotropin-releasing hormone (TRH), its analog RGH 2202, and pentetrazol and their interaction with morphine were studied in anesthesized rats. During intravenous injection or local application to the medulla oblongata, pymadin, amiridine, TRH and RGH 2202 were shown to enhance the respiratory activity of the diaphragm and to abolish its morphine-induced inhibition. TRH and RGH 2202 proved to be the most potent and safe antagonists of morphine-induced respiratory depression. These agents given in the doses sufficient to completely abolish morphine-induced respiratory depression unchanged its antinociceptive activity. Pentetrazol in the tested dose range failed to increase diaphragmatic respiratory activity or to eliminate its depression induced by morphine.