The dipeptide sweetener aspartame (N-L-alpha-aspartyl-L-phenylalanine, 1-methyl ester; alpha-APM) is relatively stable in dry powder form. However, when exposed to elevated temperature, extremes of pH and/or moisture, alpha-APM is converted into a variety of products. In aqueous solution alpha-APM decomposes to yield methanol, two isomeric forms of L-aspartyl-L-phenylalanine (Asp-Phe) [alpha-Asp-Phe and beta-Asp-Phe], and APM's diketopiperazine cyclo-Asp-Phe. Depending on beverage storage conditions, individuals drinking alpha-APM-sweetened beverages may consume small quantities of these three compounds. Relatively little has been published about the metabolism of beta-Asp-Phe and cyclo-Asp-Phe. We compared the absorption and metabolism of alpha-Asp-Phe, beta-Asp-Phe, and cyclo-Asp-Phe with that of L-phenylalanine (Phe) in adult rats. Steady-state perfusion studies of rat jejunum indicated rapid carrier-assisted uptake of Phe and alpha-Asp-Phe, but only slow passive diffusion of beta-Asp-Phe and cyclo-Asp-Phe from the lumen. Homogenates of rat intestinal mucosa, liver, and cecal contents, as well as homogenates of pure cultures of Escherichia coli B, catalyzed the hydrolysis of alpha-Asp-Phe, but not cyclo-Asp-Phe. Homogenates of E coli and rat cecal contents, but not homogenates of rat liver or intestinal mucosa catalyzed the hydrolysis of beta-Asp-Phe.