The effect of hesperetin, naringenin and its glycoside form on the Sindbis neurovirulent strain (NSV) replication in vitro was studied. All flavanones tested were not cytotoxic on Baby Hamster cells 21 clone 15 (BHK-21). Antiviral effect was evaluated by a colorimetric assay using MTT (3-(4,5 dimethylthiazol-2-yl)-2,5-dipheyl-tetrazolium bromide) and by plaque reduction assay. Hesperetin and naringenin had inhibitory activity on NSV infection. The 50% inhibitory doses (ID(50%)) of both compounds were 20.5 and 14.9 microg/ml respectively, as established by plaque assay. However their glycosides, hesperidin and naringin did not have inhibitory activity. Implying that the presence of rutinose moiety of flavanones blocks the antiviral effect. Oxygenation on the 3' positions at the B rings on the hesperetin skeleton decrease the anti viral activity at 25 microg/ml.