Isotopic markers were developed to allow measurement of the gastric emptying times of homogeneous and nonhomogeneous gel meals. Meals containing the dietary fibers psyllium and guar gum presented as homogeneous, viscous gels while meals containing the synthetic polymer polycarbophil presented as discrete gel particle-water mixtures. Fiber meals were labeled differently than polycarbophil meals. Fiber meals were labeled with 51Cr-CM-Sephadex. The marker was uniformly suspended in meals containing at least 1% guar or 2% psyllium. In contrast, polycarbophil particles were labeled by hydrating the dried granules with saline in which Na2(51)CrO4 had been dissolved. Use of the markers to measure gastric emptying was demonstrated in dogs fitted with duodenal cannulas. Half of the fiber meals emptied from the stomach in about 40 min without significant dilution by secretions. In contrast, only 8% of the polycarbophil particles emptied by 90 min. Particle-specific labeling of polycarbophil was important because the meal effluent was diluted extensively by secretions. We conclude that 51Cr-CM-Sephadex and soluble Cr-51 may be used as meal markers for estimation of the gastric emptying times of certain homogeneous and nonhomogeneous gel-type meals, respectively.