The Gray Wolf was one of the first species to be listed as endangered (1967) under the Endangered Species Preservation Act of 1966. However, until the passage of the Endangered Species Act of 1973, there was no legal basis or process for re-introducing the Gray Wolf to Yellowstone National Park and the Greater Yellowstone Ecosystem. The Endangered Species Act obligated the U.S. Fish and Wildlife Service to develop restoration plans for each species designated as Endangered. The first recovery plan was completed in 1980 but gained little traction. In 1987, the U.S. Fish and Wildlife Service published a revised Northern Rocky Mountain Wolf Recovery Plan. that led the way to wolf reintroduction. The plan was a cooperative effort between the National Park Service, Fish and Wildlife Service, academia, state wildlife agencies and environmental groups.
In January 1995, U.S. and Canadian wildlife officials captured 14 wolves from multiple packs east of Jasper National Park, near Hinton, Alberta, Canada. These wolves arrived in Yellowstone in two shipments—January 12, 1995 (8 wolves) and January 20, 1995 (6 wolves). They were released into three acclimation pens—Crystal Creek, Rose Creek and Soda Butte Creek in the Lamar Valley in Northeast East Yellowstone National Park. In March 1995, the pens were opened and between March 21 and March 31, 1995 all 14 wolves were loose in Yellowstone.
Seventeen additional wolves captured in Canada arrived in Yellowstone in January 1996 and were released into the park in April 1996 from the Chief Joseph, Lone Star, Druid Peak and Nez Perce pens. These were the last wolves released into the park as officials believed that the natural reproduction and survival were sufficient to preclude additional releases.