Previous studies from our laboratory have shown that the opioid peptide dynorphin-(1-13), although not analgesic when given by itself, can inhibit morphine-induced analgesia in naive mice and potentiate it in morphine tolerant mice. In the present study, we examined the effect of dynorphin-(1-13) with two other dynorphin-like peptides, alpha-neoendorphin and dynorphin-(1-10) amide, on respiration. Our results show that none of the peptides studied had any significant activity on the respiratory rate in mice when given alone. However, in the presence of morphine, dynorphin-(1-13) antagonized the morphine-induced respiratory rate depression in morphine-tolerant animals; alpha-neoendorphin enhanced the morphine-induced respiratory rate depression in naive but had no effect in morphine-tolerant animals and dynorphin-(1-10) amide had no modulatory effect on the morphine-induced respiratory rate depression in either group of animals.