Patterns in body mass and molt provide useful information about how birds interact with their environment and act as potential explanatory variables in behavioral and demographic differences among sex and age classes. We collected population data for Greater Sage-Grouse (Centrocercus urophasianus) in Jackson and Moffat counties, Colorado, from 1973 to 1993 prior to consideration of federal listing of the species under the Endangered Species Act. Using spotlights and long-handled nets, we located and captured Greater Sage-Grouse while they roosted (primarily at night). Each captured bird was banded and released near the site of capture. We recorded the molt status of the primary flight feathers (1–10) and measured body mass. Most birds were captured prior to and during breeding (March–May) and during the brood-rearing period (July–September). Replacement of primary (P) flight feathers was initiated in May, starting with the first primary (P1), and was completed at the last primary (P10) by late September for adult (ASY) and subadult (SY) males, and by October for hatch-year males (young of the year, HY). Adult and subadult females did not replace primary flight feathers until after nesting was completed, starting in June. Primaries 1–3 were replaced within a week of the hen leaving the nest site, with or without chicks. Primary replacement then was similar to that of adult and subadult males and was completed before early October. A few late-nesting hens retained P10 and P9 into early October. Young of the year rarely replaced juvenal P9 and P10 in the year of hatching. Body mass of males peaked (x̄ = 3000 g for adults, and 2200 g for yearlings) in March–April when birds were primarily feeding on sagebrush and then decreased to late May and reached seasonal lows (x̄ = 2200 g) in July–August. Mass of females peaked in April (x̄ = 1640 g) and decreased to 1250 g in August.