Culex pipiens pipiens L. (Diptera: Culicidae), infected per os from a membrane feeder, transmitted West Nile virus (family Flaviviridae, genus Flavivirus, WNV) at 26 degrees C horizontally during feeding to hamsters and suckling mice and vertically to F1 progeny during egg deposition. Horizontal transmission rates increased with extrinsic incubation, with 75-100% of the females transmitting on days 16 through 25 postinfection (pi). No females deposited eggs infected with WNV after the first bloodmeal on 3-8 d pi. Females vertically transmitted WNV during egg laying after their second, third, and fourth bloodmeals on days 13-33 pi. The vertical transmission rate was 4.7%. The vertical minimal infection rate was 0.52 infected F1 specimens/1,000 specimens tested from females feeding during their second and later bloodmeals on hamsters or suckling mice. The sequence of horizontal and vertical transmission is reported. A female may transmit WNV 1) horizontally to a host during feeding and subsequently vertically to her offspring during egg laying, 2) vertically to her offspring during oviposition without prior horizontal transmission to a host, and 3) horizontally to a host without vertically transmitting the virus. These two means of transmission by Cx. p. pipiens contribute to the relatively high minimum infection rates that are reached in late summer and to the survival of virus during winter and initiation of amplification in the spring in the northeastern United States.