The synthesis and biological activities of a series of N-substituted cis-4a,5,6,7,8,8a-hexa- and cis-4a,5,8,8a-tetrahydro-2H-phthalazin-1-ones are described. It was found that compounds bearing a cycloalkyl group at the 2-position exhibit the highest PDE4 inhibitory activities (pIC(50) = 8.6-9.4). The N-cycloheptyl- and N-adamantanyltetrahydrophthalazinones (7h, 8, 10, 11) show high in vivo antiinflammatory activities after oral application. Additionally, some phthalazinones were found to exhibit potent suppression of LPS-induced TNFalpha release and show moderate potency against fMLP-stimulated production of ROS.