OBJECTIVE To evaluate the shear bond strength of acetone- or water-based primers applied with or without agitation to either wet or dry dentin. METHODS Forty-eight caries-free extracted human premolars were embedded in gypsum and flat dentin surfaces were prepared. The teeth were randomly divided in eight groups with six specimens each. After acid etching and rinsing, either a HEMA and acetone (Ac) or a HEMA and water (Wa) primer were applied to dry (D) or wet (W) dentin. The primer was placed with (Y) or without (N) agitation according to the following combinations: Group 1: D-Ac-Y; Group 2: D-Ac-N; Group 3: D-Wa-Y; Group 4: D-Wa-N; Group 5: W-Ac-Y; Group 6: W-Ac-N; Group 7: W-Wa-Y; Group 8: W-Wa-N. The primed surface was dried, covered with a bonding resin and light-cured, whereupon a composite cylinder was bonded to the dentin surface. The shear bond strength was determined after 30 days in water. RESULTS The mean and standard deviations, expressed in MPa, were: Group 1: 13.1 +/- 3.42; Group 2: 15.9 +/- 4.84; Group 3: 19.8 +/- 6.64; Group 4: 18.6 +/- 5.18; Group 5: 16.5 +/- 5.25; Group 6: 25.5 +/- 4.79; Group 7: 21.9 +/- 4.94; Group 8: 17.4 +/- 4.50. The results were analyzed using ANOVA and Waller-Duncans K-ratio t-test. The acetone-based primer gave the highest bond strength to wet dentin without agitation. When the acetone-based primer was used on dry dentin a significant decrease in bond strength occurred (P < 0.05). No significant difference (P > 0.05) was found for the water-based primer on wet or dry dentin. Agitation improved the bond strength for the water-based primer, but decreased the bond strength for the acetone-based primer.