BACKGROUND The purpose of the study described here was to determine the in vivo degradation rate of 10 percent carbamide peroxide, or CP, gel in bleaching trays. The degradation rate indicates the remaining concentration of the active agent on the facial surfaces at various intervals. METHODS The researchers fabricated bleaching trays with 0.5-millimeter reservoirs and loaded them with a 10 percent CP whitening gel. The tray was seated in place in 15 patients for six different intervals that ranged from 15 seconds to 10 hours. When the tray was removed, three samples were collected from each patient: the gel remaining in the tray; the adherent gel scraped from the teeth; and a "grab" sample from the reservoir of tooth no. 8. The researchers analyzed these samples for CP according to the method specified by the U.S. Pharmacopeial Convention. RESULTS The percentage of CP recovered decreased as the intervals increased: 87 percent at 15 seconds, 10 percent at 10 hours. Log of tray, teeth and grab samples, respectively, at 15 seconds were 0.94, 0.98 and 0.96 and at 10 hours were -0.13, -0.38 and 0.11. The first-hour degradation rate for tray, teeth and grab samples, respectively, was 2.0 times, 3.6 times and one time the rate during the next nine hours. The within-subject repeatability of the samples was excellent. CONCLUSIONS The degradation rate of CP during the bleaching process is biexponential. In the tray and teeth samples, the degradation rate was accelerated during the first hour. Further research is needed to determine the cause of this acceleration. CONCLUSIONS The active agent in CP bleaching gel is available in bleaching trays for more than 10 hours. After two hours, more than 50 percent of the active agent is available, and 10 percent is available after 10 hours.