The present study accents on the relationship between dicing, nuclease stability, and RNAi activity of various types of chemically modified symmetric and asymmetric dsRNAs, covalently bound with amino-groups or cholesterol at one or both terminals. All modified dsRNAs were subjected to cleavage by recombinant Dicer enzyme. They possessed a high resistance to nuclease degradation in cell cultured medium and an excellent RNAi activity in viable cells. The best stability and RNAi activity was detected for 5'-sense amino-modified RNAs. These modifications manifested also a high long-term gene silencing effect within seven days post-transfection, while the RNAi activity of the native 21nt siRNA expired within two days. The conjugation of dsRNA with cholesterol at 5'-sense end resulted in easy intracellular delivery without transfection reagents. After a direct transfection in cells, the cholesterol-conjugated 27nt dsRNA possessed a higher RNAi activity than chole.sterol-conjugated 21nt siRNA. (c) 2007 Elsevier Inc. All rights reserved.