Eight paternal half-sib Angus calves born in late April and early May, 1988 were used to investigate the potential relationship of serum IGF-I concentration with photoperiod and various weather variables including minimum, maximum and average air temperatures, relative humidity, precipitation, wind speed and solar radiation. To determine IGF-I concentration, blood samples were obtained at birth and then weekly until the calves reached 1 mo of age and bi-weekly thereafter. Blood sampling continued until the calves reached puberty as determined by progesterone and testosterone assays. Photoperiod and each weather variable were averaged over the 3 d prior to and including the day of blood sampling (4-d average). Data were divided into two periods: (1) birth through the end of the postweaning period and (2) postweaning period only. Serum IGF-I concentrations were analyzed using a model which included the fixed effects of sex and sample number, the random effect of calf nested within sex and the fixed interaction of sex x sample number, in addition to covariates for weight, photoperiod and weather variables. From birth through the end of the postweaning test, none of the weather variables or photoperiod had significant effects on serum IGF-I concentrations when each was fitted separately. For the postweaning period only, cubic regression coefficients for minimum and average temperatures were .0962 +/- .0325 ng/ml/degrees C3 and .0976 +/- .0272 ng/ml/degrees C3, respectively (P < .01). The quadratic regression coefficient for relative humidity during the postweaning period was -.2991 +/- .1142 ng/ml/%2 (P < .05). The quartic regression coefficient for wind speed during the postweaning period was -36.435 +/- 13.00 ng/ml/(km/hr)4 (P < .01). Maximum temperature, precipitation, solar radiation and photoperiod did not have significant effects on postweaning serum IGF-I concentrations. Based on these data, we conclude that temperature, humidity and wind speed were contributing factors to variation in serum IGF-I concentrations in postweaned beef calves.