A straight runway was used to monitor changes in measures of food motivation and food consumption in order to track and to characterise the development of satiation following administration of equianorectic doses of naloxone (5.0 mgkg-1), naltrexone (2.5 mgkg-1), dexfenfluramine (1.5 mgkg-1) and d-amphetamine (1.0 mgkg-1). Naloxone and naltrexone did not reduce motivational measures or block food consumption during the early trials. These drugs brought about a prompt cessation of intake only after some food had been consumed. Dexfenfluramine displayed an early effect on motivation and hastened the onset of satiation. The anorexic activity of d-amphetamine was virtually abolished. These results indicate that the runway is a useful device for analysing the effects of drugs on eating motivation. This study has further characterised the anorexic actions of naloxone and naltrexone; the profiles of these agents can be distinguished from both dexfenfluramine and d-amphetamine. The suppressive action on food intake exerted by these particular opioid antagonists appears to arise from an intensification of the feedback from food ingestion. The mechanisms through which this effect is achieved are not known.