California State Route 121 is a state highway located in California, primarily serving the Napa Valley region. It runs approximately 24 miles, connecting Napa to Sonoma County. The route is known for its scenic views, winding through vineyards and rolling hills. The highway begins at State Route 29 in Napa and ends at State Route 37 near Sonoma. It provides access to various attractions, including Lake Berryessa and several wineries, making it a popular route for both locals and tourists. The road is well-maintained and offers a pleasant driving experience.