PURPOSE: To evaluate, by transmission electron microscopy, the extent of silver nitrate penetration into four polymerized resin blends with increasing degree of hydrophilicity after a period of 24 hours. METHODS: The uncured resin was polymerized in an experimental composite inlay processing chamber under a pressurized nitrogen atmosphere maintained at 551.6 KPa and light-activated for one complete cycle at 125 degrees C for 10 minutes. The resin blocks were sectioned occluso-gingivally into multiple 0.9 x 0.9 x 5 mm resin beams, and immersed in 50% basic, ammoniacal silver nitrate for 24 hours, exposed to a photodeveloping solution and prepared for transmission electron microscopy. The percentage distributions of silver deposits were compared using Kruskal-Wallis one-way analysis of variance (ANOVA) on ranks and Dunn's multiple comparison tests. RESULTS: No silver uptake occurred in the two most hydrophobic resins (Groups 1 & 2). The percentage distributions of silver deposits in the more hydrophilic Group 3 (6.23 +/- 1.35%) and 4 (5.08 +/- 1.09%) resins were significantly higher (P < 0.05) than were found in the more hydrophobic resins but Groups 3 and 4 were not significantly different from each other. The intensity of silver uptake increased with hydrophilicity of the resin blends. TEM examination revealed two types of silver deposits: fine spotted-type silver grains, and islands of interconnecting electron-dense clusters of silver grains. Phase separation of resin components was observed in Group 4 resin blends.