John Blayney, 84, left his home in a rural, gated community west of Sonoma known as The George Ranch around 2 p.m., Sonoma County sheriff's Sgt. Dave Thompson said.
He was seen around 3 p.m., but when he did not return home, his family reported him missing around 5 p.m., Thompson said.
Blayney suffers from mild Alzheimer's disease, Thompson said. Approximately 40 members of search-and-rescue teams from Sonoma, Napa, Marin and Contra Costa counties are scouring the area, which consists of rolling hills that contain horse ranches, Thompson said.
Blayney is an avid walker who likes to stroll to the top of a nearby hill and back to his home in the 3300 block of White Alder, Thompson said.
The search began around 6 p.m. Tuesday and continued overnight, Thompson said.
Blayney was last seen wearing navy blue sweatpants, a rust-colored, short-sleeve shirt, a dark green fleece pullover and a tan "Tilley" broad-brim hat.
He wears glasses and a hearing aid, is 5 feet 10 inches tall and weighs 140 pounds with a slim build. He is bald and has a gray beard, Thompson said.
Anyone with information on his whereabouts is asked to call the sheriff's office at (707) 565-2121.
A photo of Blayney is available at www.sonomasheriff.org.