The company is exploring expanding to Sonoma County, Rinkus said.

The app allows passengers to contact drivers who operate similarly to taxi drivers. Payment is made through a smartphone.

Drivers operate as independent contractors and are vetted through Uber. They must have clean driving records and insurance.

The company expanded into Napa to take advantage of the wine tourism industry, said Andrew Noyes, an Uber communications specialist.

"We think our Napa launch will have a significant impact on transportation for tourists and residents," he said. "Everywhere from Napa to Yountville is covered, and we plan to expand our coverage area over time."

The basic UberX service in the Bay Area comes with a $3 base fare plus 30 cents per minute and $1.50 per mile. For comparison, a San Francisco taxi charges a $3.50 base fare plus 55 cents per minute and $2.75 per mile.

First-time riders will receive $15 off their first two rides in Napa County, Noyes said.

Uber and other ridesharing services like Lyft and SideCar have faced opposition from taxi and limousine drivers who say they operate unlicensed cab services and undercut their business.

In September, the California Public Utilities Commission stepped in to regulate the ridesharing industry, creating a new category of service called Transportation Network Companies.