SR 129 begins at the Oregon state line near Flora, Oregon, as a continuation of OR 3, which travels south towards Enterprise, the seat of Wallowa County, Oregon. The highway travels north along the east side of Buford Canyon and makes a switchback turn to the south to cross Buford Canyon on the north side of the state line. SR 129 then makes another switchback turn and travels north along the west side of Buford Creek, following the canyon below Buford Ridge. After approximately , the highway crosses over the Grande Ronde River and enters the bottom of Rattlesnake Canyon, which it follows northeast towards a fork in the stream. SR 129 then turns south at a switchback and begins ascending from the canyon floor on Rattlesnake Grade, making nine turns and reaching an elevation of .