Nestling mourning doves and house finches produced elevated viremias after inoculation with 2-3 log10 plaque-forming units (PFU) of St Louis encephalitis (SLE) virus and infected 67 and 70% of Culex tarsalis Coquillett that engorged upon them, respectively. Mosquito infection rates as well as the quantity of virus produced after extrinsic incubation increased as a function of the quantity of virus ingested and peaked during days 3-5 postinoculation in mourning doves and days 2-4 in house finches. Only female Cx. tarsalis with body titers > or = 4.6 log10 PFU were capable of transmitting virus. Overall, 38% of females infected by feeding on mourning doves and 22% feeding on house finches were capable of transmission. The quantity of virus expectorated was variable, ranging from 0.8 to 3.4 log10 PFU and was greatest during periods when avian viremias were elevated. Our data indicated that nestling mourning doves and house finches were competent hosts for SLE virus and that the quantity of virus ingested from a viremic avian host varies during the course of the infection and determines transmission rates by the mosquito vector.