Crossbred (Chester White X Yorkshire X Duroc) boars were used to evaluate the effects of feeding a progestogen (altrenogest) on body growth, endocrine function (determined during feeding and after withdrawal of altrenogest), carcass composition, boar odor and testicular function (determined after a 30-d withdrawal from altrenogest). Boars from 18 litters were assigned at 12 wk of age to three treatments: 1) 18 control boars; 2) 18 boars fed altrenogest (20 mg/day) for 6 wk from 15 to 21 wk of age, followed by 30 d with no treatment; and 3) 18 boars castrated at 2 wk of age (barrows). Daily gains were greater (P less than .05) in boars fed altrenogest than in barrows through 21 wk of age but were lower (P less than .05) than those of control boars and barrows during the 30-d withdrawal period. Boars fed altrenogest weighed less (P less than .05) than control boars and barrows at 25 wk of age (at slaughter). Both groups of boars were similar in percentage of muscle and had less (P less than .05) backfat than barrows, whereas control boars had the largest (P less than .05) loineye areas. Based on evaluations by a trained sensory panel, intensity of boar odor in fat samples was similar for both groups of boars and was greater (P less than .05) than that for barrows. Weights of accessory reproductive glands and weight and sperm content of testes and epididymides were reduced (P less than .05) in boars fed altrenogest.(ABSTRACT TRUNCATED AT 250 WORDS)