Objective: To evaluate the bond strength of adhesive systems applied to dentin in vitro and in vivo, using a local anesthetic with or without vasoconstrictor. Methods: Eighteen pairs of sound human premolars scheduled for extraction due to orthodontic reasons were selected. Deep class I cavities were prepared and restored in vivo, immediately before the extraction (12 pairs) or in vitro, 6 months after the extraction and storage in water at 4^o C (6 pairs). Mepivacaine was used as local anesthetic agent, containing or not a vasoconstrictor (noradrenaline 1:100.000). The adhesive systems Single Bond (SB) or Clearfil SE Bond (CSE) were applied according to the manufacturers' directions followed by a resin restoration. Beams with a cross-sectional area of 1.0 mm² were produced and subjected to a tensile force at a cross-head speed of 5.0 mm/min. The data were analyzed with ANOVA and Tukey's test at a confidence level of 95%. Results: While in vitro SB produced significantly higher bond strengths than CSE (p>.05), 32.5±7.7 MPa and 22.6±8.6 MPa respectively, in vivo both dentin bonding systems produced similar results. Single Bond was the only adhesive affected by the experimental conditions. Although no statistically different bond strength means (p>.05) were obtained when SB was applied in vitro (32.5±7.7 MPa) and in vivo using a vasoconstrictor (27.7±10.7 MPa), the local anesthesia with a vasoconstrictor-free anesthetic significantly reduced the bond strength of SB (24.4±9.4 MPa). For this group, a statistically significant positive correlation (r=0.57) between the remaining dentin thickness and bond strength was observed. Conclusion: Bond strengths obtained in vitro using hydrated dentin may be comparable to bond strengths obtained in vivo when a local vasoconstrictor-containing anesthetic is used.

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