Mounting behaviour of uncastrated male pigs may lead to injuries and affect their welfare. The goal of this study was to reduce boars' sexual behaviour using a plant, chasteberry (Vitex agnus castus), known to alter the reproductive function of females and assumed to lower the libido of males. Pigs were fattened in groups of 21-24 under organic-production conditions. Two replicates (R1 and R2) were conducted. In each, a group of uncastrated males received 300 g of chasteberry fruit powder on top of the feed (treatment G) each morning, whereas the other group did not (treatment T). Treatments were applied from 124-158 d of age (80- 114 kg liveweight) on average in R1 and 115-173 days of age (68-117 kg liveweight) in R2. Blood samples were collected 26 (R1) or 55 (R2) d after starting the treatment on 18 or 21 pigs/treatment replicate. Animals were filmed, and sexual behaviour was recorded before treatment at the group level and individually at about 5 months of age (29-37 d after starting the treatment) for 18 pigs/treatment/replicate on two different days for 3 hours in the morning and 3 hours in the afternoon. Growth rate was similar in both treatments during the treatment period (P > 0.10). The numbers of mounting behaviours were lower (P < 0.05) in G than T pigs in R1 (P < 0.01) but not in R2 (P > 0.10). Plasma oestradiol was similar in both treatments (P > 0.10), but plasma testosterone was lower (P < 0.05) in G pigs. New groups of animals and different doses of the plant must be evaluated to draw fuller conclusions.