The seats are Madhubani, Jhanjharpur, Madhepura, Darbhanga, Samastipur (SC, Reserved), Begusarai and Khagaria - all in the Kosi river belt.
Prominent among the candidates are JD(U) president Sharad Yadav from Madhepura, being challenged by RJD's Rajesh Ranjan alias Pappu Yadav, former Union minister and BJP leader Hukumdeo Narayan Yadav from Madhubani, cricketer-turned BJP leader Kirti Azad from Darbhanga and his challenger former union minister and RJD leader M A A Fatmi.
Five women are also in the fray. An electorate of 1.12 crore electors would exercise their franchise in 10,521 polling stations.
Polling would end at 4:00 PM in the three assembly segments of Mahisi in Madhepura and Simri Bhakhtiyarpur and Aluali in the Khagaria seat.
Repoll would also be held in Sahebpur Kamal Assembly seat in Begusarai district. The repoll was necessitated by former minister Parveen Amanullah's resignation from the Nitish Kumar cabinet to move to AAP.