Preheat oven to 325 degrees.
Place tenderloin in a lightly oiled 9- x 13-inch baking pan. Melt the butter in a small saucepan over medium heat; stir in rosemary and garlic.
When the mixture is fragrant, brush it generously over the tenderloin; bake for 1 hour and 15 minutes or until cooked through. Allow tenderloin to rest for 10 minutes then slice and serve.