Determining the cause of failure to ejaculate sperm can be a diagnostic dilemma. The first diagnostic step is to ascertain whether the stallion is ejaculating. If the stallion appears to ejaculate, but there is azoospermia (absence of sperm in the seminal fluid), testing alkaline phosphatase (ALP) activity in seminal plasma can determine whether testicular and epididymal fluids are present. If ALP activity is low, the possibility of either blockage to sperm outflow in the excurrent duct system or retrograde ejaculation should be pursued diagnostically. If ALP activity is high, the possibility of a testicular defect should be pursued diagnostically. In some cases (notably plugged ampullae or transient, thermally induced testicular degeneration), treatment or the passage of time may restore a stallion's fertility.