The series of 6,7,8,9-tetrahydro-5H-5-(2'-hydroxyphenyl)-2-(4'-substituted benzylidine)thiazolo(2,3-b)quinazolin-3(2H)-ones (4a-j) and 6,7,8,9-tetrahydro-5H-5-(2'-hydroxyphenyl)-2-(4'-substituted benzylidine)-3-(4-nitrophenyl amino)thiazoloquinazolines (5a-j) were synthesized by the reported method and evaluated for their phosphodiesterase inhibitory activity. All test compounds exhibited good activity. The structure-activity relationships were also studied. In both series of compounds, electron-withdrawing substitutions showed higher activity. Among the tested compounds, 6,7,8,9-tetrahydro-5H-5-(2'-hydroxyphenyl)-2-(4'-fluorobenzylidine)-3-(4-nitrophenylamino)thiazoloquinazoline (5e), 6,7,8,9-tetrahydro-5H-5-(2'-hydroxyphenyl)-2-(4'-nitrobenzylidine)-3-(4-nitrophenylamino)thiazoloquinazoline (5j) and 6,7,8,9-tetrahydro-5H-5-(2'-hydroxyphenyl)-2-(4'-chlorobenzylidine)-3-(4-nitrophenylamino)thiazoloquinazoline (5f) were found to be more potent than theophylline (IC50 in mmol L-1 of 1.34 ± 0.09 for 5f, 1.44 ± 0.02 for 5e, 1.52 ± 0.05 for 5jvs. 1.72 ± 0.09 for theophylline). U radu je opisana sinteza serije 6,7,8,9-tetrahidro-5H-5-(2'-hidroksifenil)-2-(4'-supstituiranih benzilidin)tiazolo(2,3-b)kinazolin-3(2H)-ona (4a-j) i 6,7,8,9-tetrahidro-5H-5-(2'-hidroksifenil)-2-(4'-supstituiranih benzilidin)-3-(4-nitrofenilamino)tiazolokinazolina (5a-j) prema objavljenoj metodi te ispitano njihovo inhibitorno djelovanje na fosfodiesterazu. Svi testirani spojevi pokazuju dobro djelovanje. Proučavan je i odnos strukture i djelovanja. U obje serije spojeva, elektron-odvlačeći supstituenti doprinose jačem djelovanju. Među ispitivanim spojevima pronađeno je da 6,7,8,9-tetrahidro-5H-5-(2'-hidroksifenil)-2-(4'-fluorobenzilidine)-3-(4-nitrofenilamino)tiazolokinazolin (5e), 6,7,8,9-tetrahidro-5H-5-(2'-hidroksifenil)-2-(4'-nitrobenzilidine)-3-(4-nitrofenilamino)tiazolokinazolin (5j) i 6,7,8,9-tetrahidro-5H-5-(2'-hidroksifenil)-2-(4'-klorobenzilidin)-3-(4-nitrofenilamino)tiazolokinazolin (5f) imaju jače djelovanje od teofilina (IC50 u mmol L-1 1,34 ± 0,09 za 5f, 1,44 ± 0,02 za 5e, 1,52 ± 0,05 za 5j nasuprot 1,72 ± 0,09 za teofilin).