The objective of this study was to determine whether injections of Cloprostenol (PGF2alpha analog) in low libido Holstein bulls can improve seminal characteristics and libido. Ten low libido Iranian Holstein bulls were randomly assigned to two groups and received; (1) 250 microg ofCloprostenol (n=5) or (2) 2 mL of saline (n=5) 30 min prior to collection of semen 2 days per week for 2 months. Reaction time was significantly decreased in treatment group. Duration of ejaculation was significantly increased in treatment group. Semen volume and sperm concentration were greater in treated bulls in compare with controls. The percentage of morphologically normal sperm cells, percentage of live sperm cells, motile sperm cells and post-thaw motile sperm cells were not affected by treatment. Plasma testosterone concentrations were increased approximately two fold after Cloprostenol injection. Overall, injection of Cloprostenol at this dosage and frequency increased libido, semen volume, sperm concentration and plasma testosterone concentration in low libido Holstein bulls.