To determine whether gases generated during the breakdown of accumulated manure in an environmentally regulated building affects the onset of puberty in gilts, 42 crossbred gilts, born from 1 o t 15 September 1985, were reared indoors from 10 to 30 weeks of age on concrete slats over a pit that was drained and refilled with clean water biweekly (clean group). Forty-two gilts were reared over a pit where manure was allowed to accumulate (control group). These two groups had similar feeding, water, floor space, lighting and room temperature. A third group of 42 crossbred gilts was reared from 10 to 30 weeks of age in an open-front building with a concrete apron (outdoor group). Concentration of aerial ammonia in the control environment was three- to five-fold higher than in either the clean or the outdoor environment (P < 0·001). Average daily gain and food conversion efficiency were similar for the two indoor treatment groups, but the outdoor group gained less weight than either indoor group (P < 0·05). At 26 weeks of age all gilts were exposed to mature boars daily for 2 weeks, then every 2nd day for an additional 2 weeks. A greater proportion of gilts in the clean environment attained puberty 7 days (P < 0·05) and 10 days (P < 0·07) after first exposure to boars. These data suggest that odorous gases, such as ammonia, in the air of environmentally regulated buildings may diminish the stimulatory influence of boars on the onset of puberty in gilts.