The objective of this research was to use a model of unilateral and bilateral occlusion of the ductus deferens in the dog to study the use of alkaline phosphatase (AP) as an indicator of tubular patency. Seven healthy cross bred dogs weighing 10-15 kg BW with normal spermiogram and AP concentrations in semen were used. From each dog, three semen samples were obtained before (intact) and after right (unilateral) and left (bilateral) vasectomy. The AP concentrations were measured in duplicates by a colorimetric method in each of the three fractions (first, second (sperm-rich), third) of each ejaculate. In addition, a macroscopic and microscopic evaluation of each ejaculate was carried out to assure its quality. Data were analysed by least squares analysis of variance using SAS(R). In intact and unilateral vasectomized dogs, 96.6% of AP measured in semen corresponded to the second sperm-rich fraction whereas 1.53 and 1.83% corresponded to the first and third fractions respectively. Total AP concentrations (first and second and third fraction) in vasectomized dogs were lower than in intact animals (19.857 vs 2284.431 +/- 4.347 UAL; p < 0.001). AP concentrations were much lower in bilateral than in unilateral vasectomized dogs (142 vs 39.572 +/- 4.347 UL, p < 0.001). In summary, AP concentrations in semen can be used as an early indicator of unilateral or bilateral lack of patency of the epididymal and deferent ducts in the dog.