Purpose: This study investigated the cytotoxicity of various types of dental casting alloys after they had been conditioned in distilled water. Materials and Methods: The casting alloys investigated included one high-noble alloy (Bioherador N) and six base-metal alloys, including four Ni-Cr alloys (Remanium CS, Heranium NA, Wiron 99, CB Soft), one Co-Cr alloy (Wirobond C), and one Cu-based alloy (Thermobond). Ten disks from each alloy were conditioned in distilled water at 37°C for either 72 or 168 hours. The cytotoxicity of the alloys was then tested on Balb/C 3T3 fibroblasts, which were exposed to the alloys for 3 days at 37°C. Cell viability was determined by the MTT method. The data were analyzed by ANOVA, and follow-up comparison between the groups was carried out using Tukey and ttests. Results: ANOVA revealed a significant effect of alloy type and conditioning time ( P .001). Bioherador N was significantly less toxic than all the other alloys in the 72-hour conditioned group. After 168 hours of conditioning, its cytotoxicity was not different ( P> .05) from that of Remanium CS, Wiron 99, and Wirobond C. Thermobond and CB Soft were significantly more toxic than the other alloys at both conditioning times. Conclusion: Conditioning of base-metal alloys, other than those containing Cu, for 168 hours in distilled water makes their cytotoxicity levels comparable to that of the high-noble alloy.