Salvinorin (Sal) A is a naturally occurring, selective opioid receptor (KOPR) agonist with a short duration of action in vivo. Pharmacological properties of a C(2) derivative, 2-methoxymethyl (MOM)-Sal B, were characterized. MOM-Sal B bound to KOPR with high selectivity and displayed ϳ3-fold higher affinity than U50,488H [(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methanesulfonate] and Sal A. It acted as a full agonist at KOPR in guanosine 5Ј-O-(3-[ 35 S]thio)triphosphate binding and was ϳ5-and ϳ7-fold more potent than U50,488H and Sal A, respectively. In Chinese hamster ovary cells stably expressing KOPR, all three agonists internalized or down-regulated KOPR to similar extents, with MOM-Sal B being the most potent. In mice, MOM-Sal B (0.05-1 mg/kg s.c.) caused immediate and dose-dependent immobility lasting ϳ3 h, which was blocked by norbinaltorphimine. In contrast, ambulation in a Y-maze was increased when rats received MOM-Sal B (1-5 mg/kg s.c.). In addition, MOM-Sal B (0.5-5 mg/kg i.p.) produced antinociception (hot-plate test) and hypothermia in a dose-dependent manner in rats. MOM-Sal B was more potent than U50,488H in both tests and more efficacious than U50,488H in the hot-plate test. These latter two in vivo effects were blocked by norbinaltorphimine, indicating KOPR-mediated actions. Sal A at 10 mg/kg elicited neither antinociception nor hypothermia 30 min after administration to rats. In summary, MOM-Sal B is a potent and efficacious KOPR agonist with longer lasting in vivo effects than Sal A.Activation of the opioid receptor (KOPR), one of three major types of opioid receptors, produces many effects including analgesia, dysphoria, antipruritis, water diuresis, and hypothermia (Liu-Chen, 2004). Many selective nonpeptide KOPR agonists have been synthesized; most are arylacetamide compounds, including U50,488H, U69,593, ICI 204,448, and asimadoline. Nalfurafine is an exception, being an epoxymorphinan.Chewing or smoking the leaves of Salvia divinorum or drinking juice of crushed leaves causes hallucinations in humans (Siebert, 1994). Salvinorin (Sal) A (Fig. 1), a neoclerodane diterpene, was isolated and identified as the main active ingredient (Valdés, 1994). In a large scale screening of G protein-coupled receptors, transporters, and ion channels by radioligand binding assays, Roth et al. (2002) found that Sal A was a highly potent and selective agonist for the KOPR. Since then, several pharmacological studies on Sal A have been performed. Sal A binds to KOPR with a high affinity similar to U50,488H, a prototypic selective KOPR agonist. It does not show significant affinities to and ␦ opioid receptors or the nociceptin/orphanin FQ receptor . Sal A is reported to be more selective for KOPR than U50,488H or U69,593 (Beguin et al., 2008). Sal A acts as a very potent full agonist at KOPR in [35 S]GTP␥S binding, intracellular calcium mobilization, and potassium conductance assays (Chavkin et al., 2004;Wang et al., 2005). Sal A