The objective of this study was to evaluate the effect of Eqvalan (ivermectin) on stallion semen quality and freezability. Experiments were performed using 22 Freiberger stallions, randomly divided into a control and test group. Semen was collected once a week for 17 weeks from October 1997 to February 1998. Eqvalan was given orally to all stallions of the test group at a therapeutic dose of 0.2 mg ivermectin/kg. Besides measuring the scrotal width, ejaculates were collected to determine the volume, concentration, and the motility and morphology (normal sperm, major defects, vacuoles and acrosome defects). In addition, the motility and viability (fluorescence staining with propidium iodide/SYBR-14) were tested in all frozen-thawed semen samples. During the experimental period, stallions treated with Eqvalan had significantly better concentration (P < 0.0001) and motility (P < 0.0001) in fresh semen compared to control animals. After freezing-thawing, the motility (P < 0.0001) and viability (P = 0.0003) of semen also increased significantly for treated stallions. Regarding morphology, normal sperm (P < 0.0001), major defects (P = 0.0027) and vacuoles (P = 0.0236) were significantly better in the Eqvalan group. In addition to group differences we also observed a time effect on morphological parameters as shown by a decrease of normal sperm and an increase of major defects in both groups during the experiment. Our results demonstrate that a single oral application of Eqvalan did not negatively influence the quality and freezability of stallion semen in the nonbreeding season. Rather, it seems that Eqvalan has a favorable effect on stallion fertility as most sperm parameters examined were significantly improved in treated animals compared to control.