The same or closely related stallions are used for reproduction across studs differing in environments. Stallions may perform differently across studs, and adjustment for the environmental sire x stud effect (s) may be recommended. However, there are no available studies analysing the influence of such effect on estimation of genetic parameters in horses. This research aimed at assessing the effect of including s effects in the models for eight body measurements: height at withers (HW), height at chest (HC), leg length (LL), body length (BL), width of chest (WC), heart girth circumference (HGC), knee perimeter (KP) and cannon bone circumference (CBC). Using data from 16,472 individuals, genetic parameters were estimated using a multivariate animal model by REML. The estimated heritabilities ranged between 0.26 (HC and WC) and 0.57 (HW) and estimates for the s effect from 0.02 (HW) to 0.09 (WC and HGC). Correlations between s effects were positive, significant and from moderate to low, varying between 0.09 (HW-BC) and 0.62 (HW-BL); except for LL, which were negative and low (-0.20 - -0.68). Selective decisions or preferential management practices can cause this environmental effect. The inclusion of an s effect in models fitted for genetic evaluation in horse breeding schemes can be advisable.