Ghrelin is the only circulating hormone known to stimulate food intake. However, a paucity of good pharmacological tools has prevented a full understanding of the role of ghrelin, and its cognate receptor GHSr1 in the physiological control of feeding. We have identified a series of GHSr1 ligands with varied pharmacology, including full and partial agonists and inverse agonists, with PK properties suitable for in vivo use. We have used these to probe the role of GHSr1 in the normal diurnal control of food intake in mice. Freely feeding mice (C57Bl6J Ola/Hsd) were orally dosed with GHSR1 ligands at the beginning of the dark phase, when feeding behaviour was most active. Doses were chosen to achieve unbound plasma and/or brain concentrations above the in vitro IC50/EC50 at the GHSr1 receptor. Food intake was measured at intervals over the following 24-hours.GHSr1 partial agonists stimulate food intake in the first hours after administration, and this increase in cumulative food intake is maintained throughout the diurnal cycle, with no compensatory underfeeding at later time points (2hr, 197% cf. vehicle-treated control; 6hr, 132%; 24hr, 119% (all p<0.05)). Conversely, a CNS-penetrant ghrelin inverse agonist decreased food intake in the time period shortly after administration, but this decrease was not maintained, and compensatory overfeeding during later phases of the diurnal cycle meant that there was no overall effect on 24-hr food intake (2hr, 63% (p<0.05); 6hr, 80% (p<0.005); 24hr, 94% p=NS). This was not due to drug exposure, since a second administration of compound was unable to extend the period during which food intake was decreased (6-8 hrs, antagonist T0 104% cf. vehicle treated control, antagonist T0, T4 104% (both NS)). Similar data was obtained using a CNS-penetrant inverse agonist identified by Abbott. Both inverse agonists were without effect in GHSr1 knockout mice (e.g. 6hr, inverse agonist, 93% (NS)).A difference was detected between the efficacy of CNS-penetrant GHSr1 inverse agonists, and those unable to cross the blood-brain barrier. Whereas the effect of CNS-penetrant lasted first 6 hours, that of an inverse agonists confined to the peripheral circulation had effects only in the first 2 hours after administration (0-2hr, 46% (p<0.005) cf. vehicle; 4-6hr, 106% (NS)). Interestingly, a brain-penetrant neutral antagonist compound had no effect on food intake alone but blocked the feeding stimulated by a GHSr1 agonist. These results suggest that, while GHSr1 activation can increase food intake, in this model, the constitutive activity of GHSr1 receptors in the brain may be more important than circulating ghrelin levels for the physiological regulation of food intake.