The semen characteristics and testicular size of 398 3-year-old maiden Dutch Warmblood stallions were studied during February and March. Mean values (+/- SD) of age (1030 +/- 88 days) and testicular size (9.8 +/- 0.9 cm) of the maiden stallions were determined as well as the following semen characteristics (mean of two ejaculates, taken 1 h apart): volume (65 +/- 26 ml), sperm concentration (2.061 +/- 1.685 x 10(8) ml-1), total number of spermatozoa (1.129 +/- 0.71 x 10(10)), percentage of progressively motile spermatozoa (68 +/- 9%), percentage of live spermatozoa with normal morphology (66 +/- 15%), total number of progressively motile morphologically normal spermatozoa (5.307 +/- 3.833 x 10(9)) and pH (7.5 +/- 0.2). The morphology of the spermatozoa was studied and data on the most common abnormalities are presented. The first and second ejaculates show significant differences (P < 0.05) in volume, motility, number of spermatozoa, sperm morphology, total number of progressively motile morphologically normal spermatozoa and pH. The original breed has been split up into two different lines. There was a significant difference (P < 0.05) in semen characteristics between the types (riding and carriage horses). The semen quality and quantity of semi-siblings were compared and a significant sire effect (P < 0.05) was shown for the following characteristics: volume, motility (%) and sperm concentration. These results indicate that there are differences of heritability of those factors, which makes it possible to select on semen quality.