Fig. 6. Effect of dietary P restriction on rate of Pi uptake in the proximal intestine (open bars) and pyloric caeca (PC; filled bars) of rainbow trout. Fish were fed either low-P (LP) or high-P (HP) diet for 20 days. Sleeves of intestine and PC were sampled from the fish at days 2, 5 and 20, and Pi-uptake analyzed in vitro. Values are means ± S.E.M. (N=5 fish, 2 tissue sleeves per fish assayed). The tissue incubation medium contained 0.1 mmol l^–1 Pi. At this Pi concentration, the active component represents 75% of total Pi uptake in PC (see Fig. 3). Between LP and HP fish, Pi-uptake was not different at days 2 and 5 in intestine and PC; however, at day 20, the uptake was markedly different between LP and HP fish in the intestine (*P=0.002), but not in PC (NS; P=0.35). Among sampling days (day 2, 5 and 20) in each treatment, Pi uptake was markedly different in the intestine (ANOVA P=0.001 and 0.03, regression P=0.0003 and 0.007 in LP and HP fish, respectively), but not in PC (ANOVA P=0.34 and 0.95, regression P=0.14 and 0.76 in LP and HP fish, respectively).