Residents in Tullibody, Clackmannanshire, have been helping officers and the family of Brandon Iliff, aged 13 and his eight-year-old brother, Bernie, after the alarm was raised when they failed to return home at 6pm.
They had taken their bicycles for a ride around their street in the town's Redlands Road. The family searched for them in the surrounding area, but could find no trace and reported the incident to the police.
A full police investigation was underway last night to establish their whereabouts and officers appealed to anyone who can help trace them to come forward.
Brandon is described as white and wearing a white t-shirt, blue trousers and trainers with yellow tops.
Bernie is described as white with dirty blond hair. He was wearing a long-sleeved Mario t-shirt, black trousers and blue Spiderman trainers.
Superintendent Matt Richards said: "Given the boys' young age and the time since they were due to return home we are growing increasingly concerned for their well being.
"Officers are currently conducting various lines of enquiry locally to trace Brandon and Bernie and anyone with information as to their current whereabouts is asked to contact police immediately.
"I would also ask members of the public in the Tullibody area to check their sheds, garages and other outbuildings where two children might take refuge."
The street lies on an estate in the centre of Tullibody, close to a busy main Stirling and the River Cambus, a tributary of the River Forth.
Those with information can contact Police Scotland on 101.