Phthalates are diesters of phthalic acid and an alcohol moiety. Phthalates have been classified as endocrine disruptors and have a broad range of effects with unknown mechanisms. Some of the effects of phthalate are consistent with disruptions of normal glucocorticoid homeostasis, and in particular, with defective function of 11beta-hydroxysteroid dehydrogenase 2 (11beta-HSD2). In the present study, we tested 12 phthalate diesters and four monoesters for the inhibition of human and rat kidney 11beta-HSD2. We examined the modes of inhibition and looked for a relationship between the potency for inhibition and the chemical structures. Of the phthalate diesters we tested, dipropyl phthalate (DPrP) and di-n-butyl phthalate (DBP) significantly inhibited both human and rat 11beta-HSD2 activities. The IC(50)s were 85.59 microM for DPrP and 13.69 microM for DBP when calculated for rat 11beta-HSD2. As diesters, 8 of the phthalates did not affect 11beta-HSD2 enzyme activity. Compared to the diesters that were inhibitory, the 8 non-inhibitory phthalates, had either fewer carbons, that is 1 or 2 carbons in the alcohol moiety, or more carbons, 5-10, as a branched or unbranched chain in the alcohol moeity. However, phthalates could be inhibitors with six carbons in the alcohol moiety if the carbons were cyclized, as in dicyclohexyl phthalate (DCHP), which inhibited rat 11beta-HSD2 with an IC(50) of 32.64 microM. Thus, whether a phthalate is an inhibitor may reflect the size and shape of the compound. Although the diesters are the compounds used in manufacturing and present as environmental contaminants, it is the monoester metabolites that are detected in human serum and urine. We showed that mono (2-ethylhexyl) phthalate (MEHP) significantly inhibited human (IC50)=110.8+/-10.9) and rat (121.8+/-8.5 microM) 11beta-HSD2 activity even though its parent compound, di(2-ethylhexyl) phthalate (DEHP) did not. MEHP was a competitive inhibitor of 11beta-HSD2 enzymatic activity. We conclude that phthalates of a certain size act as competitive inhibitors.