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Aim: To investigate the release of Ca2+ ions from experimental composite materials with the addition of 1-10% copper-doped mesoporous bioactive glass nanospheres (Cu-MBGN). Materials and Methods: Four experimental composites based on a light-curable Bis-GMA/TEGDMA (60:40) resin matrix with 1, 5, and 10 wt% Cu-MBGN were prepared. A material containing 10% conventional bioactive glass 45S5 was prepared as a reference. All materials contained a total filler load of 70 wt%. The concentrations of Ca2+ released from the experimental composites were measured by the Arsenazo III method using a dual-beam UV-Vis spectrophotometer (Genesys 180, Thermo Fisher Scientific, Waltham, MA, USA) calibrated with standard solutions containing 0.05-5 mg/L Ca2+. Light-cured specimens of the experimental composites (d = 6 mm, h = 2 mm) were immersed in 3 mL of distilled water for 24 hours, after which their eluates were analyzed. Subsequently, the samples were again immersed in 3 mL of distilled water for 7 days and the concentration of Ca2+ released was measured again. Results: The highest concentration of Ca2+ was measured after one day in the eluate of the material containing 10% conventional bioactive glass (1.41 mg/L), while the concentrations of Ca2+ measured after 7 days were lower (0.30 mg/L) and were statistically similar to the values measured for the material with 1%Cu-MBGN after 7 days of immersion (0.17 mg/L). Lower Ca2+ values that were statistically similar for the 1-day and 7-day time points were measured for the materials with 1% (0.17-0.24 mg/L) and 5% Cu-MBGN (0.39-0.42 mg/L) Cu-MBGN. The material with 10% Cu-MBGN was the only one that showed an increase in Ca2+ release after 7 days (0.71 mg/L) compared to 1 day (0.35 mg/L). Experimental composite materials functionalized with copper-doped mesoporous bioactive glass nanospheres showed the ability to release Ca2+, which corresponded to the proportion of bioactive glass in the material. Conclusion: The materials showed the highest Ca2+ release during the first 24 hours, except for the material functionalized with 10%Cu-MBGN. This study was supported by Croatian Science Foundation (IP-2019-04-6183). [ABSTRACT FROM AUTHOR]