The effect of environmental enrichment on the welfare of stabled horses was estimated on the basis of positive and negative behavioural indicators. Six stabled horses were exposed for seven days to each of two conditions in early spring: i) a window at the back of the loose box was opened, but no horses were within the view of the subject horses (OW) and ii) the window in the loose box was opened and two horses that had been in the same stable as the subject horses were turned out to the paddock next to the stable and were within view of the subject horses (OWH). The window in the loose box had been closed prior to the start of the study for protection against the cold during winter months (CW condition) but horses could see outside the loose box through the grille door. The behaviour of the subject horses was recorded by video camera from 1300 to 1530h, firstly, in the CW condition for the three days prior to treatment as the control condition, and then for the last three days of each week in the OW and OWH conditions. The behaviour was focal- and instantaneous-sampled at 30-s intervals. Significant differences between the effects of the loose box conditions on the mean percentage of time spent in standing behaviour, looking behaviour, and bedding investigation behaviour (which may be an indicator of frustration) were observed but no significant difference in the mean percentage of time spent in standing-sleep behaviour (which may be an indicator of behavioural satisfaction) was observed. When the window was opened, bedding investigation and standing behaviour decreased and, when the conspecifics were within view, bedding investigation behaviour decreased and looking behaviour increased. The results suggested that the OW and OWH treatments suppressed the frustration of stabled horses which did not perform any abnormal behaviour, but may not increase the behavioural satisfaction of stabled horses.