Cryopreservation compromises sperm fertilising capacity due to a series of alterations in sperm structure and physiology; use of antioxidants such as melatonin, added to freezing media, may help to reduce sperm cryoinjury. To test the effect of melatonin on Bulldog [Canis lupus familiaris] sperm cryosurvival, spermatozoa were diluted in a standard freezing medium, cooled to 5°C, and added more freezing medium to obtain 200 x 106 cells/mL, and 5% glycerol. Diluted spermatozoa were added melatonin (0.0, 0.0005, 0.002, and 0.0035 mol/L), and packaged in 0.25 mL straws which were further cooled to -5°C before freezing in liquid nitrogen. Thawing was carried out at 70°C for 5 sec, and then spermatozoa (at 37°C) were assessed for progressive motility, viability, plasma membrane integrity, acrosome integrity, capacitation status, and plasma membrane fluidity; data was analysed by ANOVA to detect differences between melatonin doses. There were differences (P<0.05) in the percentage of sperm having hyper-fluid membranes, intact acrosome, capacitated acrosome-intact, and acrosome reacted, being the values for high (0.002 and 0.0035 mol/L) better than for low (0.0 and 0.0005 mol/L) melatonin doses. In conclusion, 0.002 and 0.0035 mol/L melatonin improved cryosurvival of sperm from Bulldog males.