Abstract 30 adult rainbow trout (0. mykiss) (202.1 ± 3.5 g SEM) in a fresh water recirculation system were fed the first test diet containing small radio-opaque glass beads 'ballotini' (0.6-0.9 mm) until all fish were satiated. The fish were re-fed at different time intervals (t= 0, 4,8, 12,24,30 and 36 hours) with the second test diet containing large size of glass beads (1.16-1.40 mm) and X-rayed. The amount of feed consumed by fish for each time interval was calculated following the quantification of the number of different size of glass beads in the developed X-ray sheets and the appetite return of trout was modelled. A first order equation best explained the appetite revival data (Feed Intake = 106.26* (1-e-0.079 time)). The times for 50 and 95 % of return of appetite in rainbow trout were observed as 8 and 28 hours, respectively. It was suggested that incorporation of markers of different sizes into various types of feed allows the amounts of each type of feed presented either simultaneously or as discrete meals offered within a 12 -hour-period.