The aim of the study was to investigate in commonly used experimental animals the pulpal wall and the dentin with special reference to the number and diameter of the dentinal tubules. The coronal dentin of a total of 26 permanent, intact teeth from rat, cat, dog and monkey was fractured experimentally. The calculations were based on the study of about 150 scanning electron microscope (SEM) photographs of fractured surfaces and of the pulp chamber wall. The number of dentinal tubules per unit area was calculated and the tubule diameter was measured at various distances from the pulp. The tubules were found to be widest at the pulpal wall, where the mean diameter ranged from 1.7 to 2.8μm, the highest value being found in monkey and the lowest in the incisors of rat. In the middle part of the dentin the mean diameter varied from 1.0 to 1.3 pm and at the periphery it was 0.6 to 0.9 pm. The largest number of tubules per unit area was found immediately incisal of the pulp horn and the smallest number cervically, near the enamel. At the pulpal wall the mean number of the tubules ranged from 50,000 to 90,000/mm2, in the middle of the dentin it was between 37,000 and 50,000/mm2and at the periphery it was between 10,000 and 25,000/mm2. In all species examined the dentinal tubules showed a straight course, except in incisors of rat, where local irregularities were seen. A comparison was also made between the species studied and homo with respect to the number and width of the dentinal tubules.