Experiments were conducted investigating the effects of prostaglandins and prostaglandin synthesis inhibitors on libido in boars. In Experiment 1, two prostaglandin products were compared with regard to expediting the training of boars for semen collection. On each of five consecutive days, boars received i.m. treatment with saline, dinoprost tromethamine or cloprostenol sodium (n=12/group). On each of day 1 (p=0.06), day 2 (p<0.05), and day 3 (p<0.05), but not on day 4 or 5 (p>0.1), the percentage of boars collected after dinoprost tromethamine, but not cloprostenol sodium, was greater than controls. In Experiments 2 and 3, libido in boars that were trained previously for semen collection was assessed after treatment with prostaglandin synthesis inhibitors, testing the hypothesis that endogenous release of prostaglandin is necessary for expression of sexual behaviors. In Experiment 2, boars treated with flunixin meglumine (n=12) had suppressed (p<0.01) levels of 15-ketodihydro-prostaglandin-F(2) (PGFM) in serum but characteristics of libido were similar (p>0.1) to controls (n=12). In Experiment 3, boars were administered indomethacin orally (n=12) or served as untreated controls (n=12). Indomethacin decreased (p<0.01) serum levels of PGFM, increased (p<0.05) the number of false mounts (mounting artificial sow but dismounting before an ejaculate was collected), and tended (p=0.09) to lengthen the interval between entering the collection pen and the start of ejaculation. These results suggest that prostaglandin synthesis and release is necessary for the complete display of normal sexual behaviors in boars.