The objective of the present study was to investigate the effects of pulpal inflammation on sensory thresholds to electrical stimulation and on pulpal blood flow in human teeth. The study was approved by the Mahidol University Ethical Committee. The experiments were done on 24 healthy premolars in 12 subjects (aged 15-25 yr.). The teeth were scheduled for extraction as part of orthodontic treatment. Sensory thresholds were determined with a monopolar electrical pulp tester (1 ms, constant current) and pulpal blood flow was recorded with a laser Doppler flow meter (Moor type MBF3D/42) after isolating the tooth with opaque black rubber dam. Dentin was exposed at the tip of the buccal cusp of a premolar on one side by cutting a cavity (diam. 3 mm, depth 3 mm) with a diamond bur in an air-rotor handpiece under a constant stream of water. The cavity was filled with gutta percha (GP) for one week. A similar procedure has been shown to produce mild pulpal inflammation due to the poor seal of the GP. The contralateral premolar was treated in exactly the same way except that the cavity was sealed with a glass ionomer material (Vitremer®, 3M; GI) after inserting the GP. Treatment for one week with both GP and GI had no significant effect on either the sensory threshold (before: mean 45.1 m A, s.d. 17.7 m A; after: 42.4 ± 12.8 m A) or pulpal blood flow (before: 4.5 ± 1.8 P.U.; after: 4.5 ± 1.4 P.U.; P> 0.05. Paired t-test); but treatment with GP alone caused both a significant decrease in the sensory threshold (from 40.4 ± 13.9 m A to 31.6 ± 10.5 m A; P< 0.001) and an increase in pulpal blood flow (from 4.4 ± 2.0 P.U. to 11.7 ± 5.0 P.U.; P<0.001.). It is concluded that the unsealed GP filling produced pulpal vasodilatation, which can be attributed to pulpal inflammation. It also caused lowering of the sensory threshold to electrical stimulation. This may have been due to either sprouting of nerve terminals in the tooth or to central sensitization in the pain pathways from the pulp.