This study aimed to investigate the effects of heavy orthopaedic forces, produced by rapid maxillary expansion, on pulpal tissue of pre-molar teeth. Teeth extracted as part of an orthodontic treatment plan, an average of 3 months, 6 months and 18 months after rapid maxillary expansion, were analysed using histopathological techniques. Control teeth were extracted, for orthodontic reasons, before any force treatment. Non-parametric tests were used to compare the results from the three treatment groups and one control group. Vessel diameter, haemorrhage, congestion and inflammatory cell infiltration varied between groups, and the differences between the control and 3-month groups, and the 3-month and 18-month groups were most significant. In conclusion, orthopaedic forces exerted by rapid maxillary expansion caused reversible vascular changes in pulpal tissue of upper pre-molar teeth.