This study examined the effects of the cannabinoid CB(1) receptor agonist (R)-methanandamide and the CB(1) receptor antagonist SR-141716 on open-field behaviors in rats. Animals were examined after administration of (R)-methanandamide (dose range 10 to 30 mg/kg) plus vehicle, and the two drugs in combination; the dose range of SR-141716 was 0.3 to 5.6 mg/kg. Injections were given intraperitoneally 20 min prior to initial testing. Additional exposures to the open-field arena occurred for the groups treated with 30 mg/kg (R)-methanandamide at 60 and 120 min post administration. There was a dose-related suppression of ambulation (horizontal activity) and rearing (vertical activity) after (R)-methanandamide administration. Coadministration of SR-141716 did not counteract the suppression induced by 10 and 18 mg/kg (R)-methanandamide but rather tended to augment it, especially with regard to ambulation using SR-141716 doses of 1 mg/kg and up. The latency to leave the starting area in the center of the field was significantly elevated by 30 mg/kg (R)-methanandamide. This effect was completely blocked by SR-141716. With increasing doses of SR-141716, there was an increase in grooming and scratching. Generally, the strongest effects occurred 20 min post administration with the exception of grooming, which reached maximum at 60 min post. Differences in the number of circlings, vocalizations, urination, and defecation generally did not differ clearly among treatments. These results coupled with previous open-field data examining combinations of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and SR-141716 [Pharmacol. Biochem. Behav. 73 (2002) 911] underscore pharmacological differences between (R)-methanandamide and Delta(9)-THC revealed by their interactions with SR-141716.