Single doses of 4.5 mg/kg of the opiate agonist, morphine, the partial agonists, cyclazocine, pentazocine, levallorphan and nalorphine, the antagonists, naloxone and naltrexone and dextrorphan were injected i.p. in groups of rats. Behaviour was noted for 30 min following injection and at this time, brains were removed and levels of noradrenaline (NA), dopamine (DA), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined spectrophotofluorimetrically. Only morphine and cyclazocine altered behaviour and 5-HIAA was the only substance which was consistently elevated by all the compounds tested. These results indicate that a causal relationship between the acute effects of opiate agonists, partial agonists and antagonists on brain 5-HT metabolism and their pharmacological actions is unlikely.