Ayahuasca is a psychoactive plant mixture typically composed of the beta-carboline-rich Banisteriopsis caapi vine and the hallucinogenic plant Psychotria viridis. Ayahuasca has long been used by aboriginal populations for its putative spiritual and medicinal benefits. Although the presumed primary chemical constituents of ayahuasca have been identified, little is known about the basic in vivo pharmacology of the extract. Two principal constituents of ayahuasca, the beta-carboline harmine and N,N-dimethyltryptamine (DMT) were selected for detailed study in mice using the Functional Observational Battery (FOB). The B. caapi extract was then examined alone and in combination with DMT. Harmine and the B. caapi extract produced similar effects in the FOB, particularly in the open field. Clonic and tonic motor movements were augmented by DMT administration. Harmine and B. caapi decreased acoustic startle amplitude without significantly affecting prepulse inhibition. DMT appeared to attenuate startle-decreasing effects of harmine and B. caapi, although these effects fell just short of significance. These results suggest that the behavioral effects of B. caapi in mice may be attributed in large part to its principal alkaloid species, harmine, and related beta-carbolines in the extract. Hence, the presence of the banisteriopsis vine in the admixture may directly contribute to the unique subjective effects of ayahuasca.