Different approaches have been investigated to reduce weaning stress in foals. The present study examines whether the presence of unrelated adult horses at weaning would reduce the social stress of weaning and the emergence of undesirable behaviours. This study was conducted in two sites. Subjects were 12 colts and 20 fillies from Arab (site 1), Anglo-Arab and French-Saddlebred (site 2) breeds. Weaning occurred outdoors at 4.5 (site 1) or 7 (site 2) months of age. At both sites, foals were divided into Peer-Weaned (PW) and Adult-Weaned (AW) groups: in the formers, foals were maintained after weaning in same-age groups; in the latters, two unfamiliar adult horses were introduced following the removal of all mares. At farm 1, one PW group (nPW1-1=5) and one AW group (nAW1-1=4) were constituted. At farm 2, four balanced groups, 2 PW groups (nPW2-1=6, nPW2-2=6) and 2 AW groups (nAW2-1=6, nAW2-2=5), were formed. Thus, three separate trials were conducted, with each trial consisting of one AW group and one PW group. PW and AW groups were kept in separate pastures throughout the study period. Levels of salivary cortisol were measured one day before to four days after the weaning day. Foals behaviours were recorded prior to weaning and through one month post-weaning. Statistical significance was tested using non-parametric tests. In all groups, weaning was followed by increased vocalization, increased locomotion and increased salivary cortisol concentration. However, signs of stress were less pronounced and shorter in duration in weanlings with adults (e.g. whinnies: P<0.05; salivary cortisol: P<0.05). Only foals without adults exhibited for several weeks post-weaning aggressiveness towards peers (P<0.05) and abnormal behaviours (P<0.05) including excessive wood-chewing and redirected sucking towards peers. In conclusion, introducing adults to minimize weaning stress in foals and later on aggressiveness and abnormal behaviours appears as the most promising approach to date.