Cisplatin chemotherapy frequently causes severe vomiting in two temporally separated clusters of bouts dubbed the acute and delayed phases. Cannabinoids can inhibit the acute phase, albeit through a poorly understood mechanism. We examined the substrates of cannabinoid-mediated inhibition of both the emetic phases via immunolabeling for serotonin, Substance P, cannabinoid receptors 1 and 2 (CB(1), CB(2)), and the neuronal activation marker Fos in the least shrew (Cryptotis parva). Shrews were injected with cisplatin (10mg/kg i.p.), and one of vehicle, Delta(9)-THC, or both Delta(9)-THC and the CB(1) receptor antagonist SR141716A (2mg/kg i.p.), and monitored for vomiting. Delta(9)-THC-pretreatment caused concurrent decreases in the number of shrews expressing vomiting and Fos-immunoreactivity (Fos-IR), effects which were blocked by SR141716A-pretreatment. Acute phase vomiting induced Fos-IR in the solitary tract nucleus (NTS), dorsal motor nucleus of the vagus (DMNX), and area postrema (AP), whereas in the delayed phase Fos-IR was not induced in the AP at all, and was induced at lower levels in the other nuclei when compared to the acute phase. CB(1) receptor-IR in the NTS was dense, punctate labeling indicative of presynaptic elements, which surrounded Fos-expressing NTS neurons. CB(2) receptor-IR was not found in neuronal elements, but in vascular-appearing structures. All areas correlated with serotonin- and Substance P-IR. These results support published acute phase data in other species, and are the first describing Fos-IR following delayed phase emesis. The data suggest overlapping but separate mechanisms are invoked for each phase, which are sensitive to antiemetic effects of Delta(9)-THC mediated by CB(1) receptors.