The widespread use of dairy bull semen in artificial insemination requires that semen production be as efficient as possible. Investigators have shown that low libido is one of the most common problems encountered with bulls and one that frequently leads to culling. In commercial situations, treatment with PGF2α has been used to expedite mounting behavior, as well as restore libido in boars displaying decreased sex drive (Szurop et al, 1985). When administered prior to ejaculation, PGF2α markedly increased the number of spermatozoa in first ejaculates from bulls and rabbits (Hafs et al, 1974). The objective of this study was to determine whether injections of PGF2α in low libido Iranian Holstein bulls can improve semen characteristics and libido.