The binding site of sn-1(3)-regioselective Rhizopus oryzae lipase (ROL) has been engineered to change the stereoselectivity of hydrolysis of triacylglycerol substrates and analogs. Two types of prochiral triradylglycerols were considered: 'flexible' substrates with ether, benzylether or ester groups, and 'rigid' substrates with amide or phenyl groups, respectively, in the sn-2 position. The molecular basis of sn-1(3) stereoselectivity of ROL was investigated by modeling the interactions between substrates and ROL, and the model was confirmed by experimental determination of the stereoselectivity of wild-type and mutated ROL. For the substrates, the following rules were derived: (i) stereopreference of ROL toward triradylglycerols depends on the substrate structure. Substrates with 'flexible' sn-2 substituents are preferably hydrolyzed at sn-1, 'rigid' substrates at sn-3. (ii) Stereopreference of ROL toward triradylglycerols can be predicted by analyzing the geometry of the substrate docked to ROL: if the torsion angle phiO3-C3 of glycerol is more than 150 degrees, the substrate will preferably be hydrolyzed in sn-1, otherwise in sn-3. For ROL, the following rules were derived: (i) residue 258 affects stereoselectivity by steric interactions with the sn-2 substituent rather than polar interactions. To a lower extent, stereoselectivity is influenced by mutations further apart (L254) from residue 258. (ii) With 'rigid' substrates, increasing the size of the binding site (mutations L258A and L258S) shifts stereoselectivity of hydrolysis toward sn-1, decreasing its size (L258F and L258F/L254F) toward sn-3.