The objectives were to evaluate factors affecting reproductive performance of dairy heifers. Holstein heifers (6389) were housed in a feed lot located in Parma, ID. Each week heifers weighing > or =290 kg were initiated in the reproductive program, which consisted of one injection of PGF(2alpha) and AI on detection of estrus. Heifers not inseminated by 11 days after the initiation of the breeding program received a second injection of PGF(2alpha). Pregnancy was diagnosed at 40+/-3 and 90+/-3 days after AI. Average daily minimum temperature (ADMnT), average daily maximum temperature (ADMxT), and average daily rainfall (ARF) were recorded between 15 days prior to and 15 days after the day of AI or the day of initiation of the breeding program. Exposure to air temperature was classified as: cold stress (CS=ADMnT< or =4 degrees C), no stress (NS=ADMnT>4 degrees C and ADMxT<29 degrees C), and heat stress (HS=ADMxT> or =29 degrees C). Exposure to rainfall was classified as above (HRF) or below (LRF) the mean for the period in question. Heifers were classified according to body weight at initiation of the breeding program as thin (TH<340 kg); moderate (MD=340-365 kg); and heavy (HY>365 kg). Service sire was associated with conception rate at 40 and 90 days after first AI. Although exposure to air temperature was not correlated with conception rate at 40 days after first AI, heifers exposed to cold stress had smaller conception rates at 90 days after first AI because they were more likely to lose pregnancy between 40 and 90 days of gestation. The proportion of heifers inseminated after initiation of the breeding program was correlated with body weight and exposure to cold stress. Exposure to cold stress was also correlated with the proportion of heifers conceiving within 11 and 22 days after initiation of the breeding program. From this study a correlation was established between body weight and rate of insemination and between the exposure to cold stress and reproductive efficiency of Holstein heifers.