Prolyl oligopeptidase (POP) is a serine protease highly expressed in the brain that hydrolyses peptide bonds at the carboxyl terminal of prolyl residues. There is evidence that this enzyme participates in several functions of the central nervous system. Scutellaria racemosa Pers demonstrated significant and selective POP inhibition. Fractionation of the hydroalcoholic extract resulted in the isolation of four main constituents identified for the first time from S. racemosa Pers, the triterpenoid lupeol (1) and the flavonoids oroxylin A (5,7-dihydroxy-6-methoxyflavone, 2), hispidulin (4',5,7-trihydroxy-6-methoxyflavone, 3), and oroxyloside (oroxylin A 7-O-glucuronide, 4). Inhibitory assays indicated that 3 and 4 at a concentration of 100 microM inhibit 43 and 34% of total POP activity, respectively.