The objective of this research was to determine the site of dissolution of viscous polysaccharides in the intestinal tract of birds fed a barley-based diets and the site of action of enzymes designed to degrade such polymers. A barley-based diet (60% Condor variety, 4.58% β-glucan) was fed with and without β-glucanase supplementation to broilers from 1-21 days of age (4 replicate pens of 5 birds per pen), at which time weight gain, feed intake and viscosity of digesta supernatant (12,500 x g, 5 min) from the gizzard, first and last half of the small intestine was measured. Two β-glucanase sources were tested, a commercial multienzyme product, blended from several microbial and fungal fermentations containing β-glucanase, xylanase and amylase activities (Avizyme Sx, Finnfeeds International Ltd., BG1) and a single Trichoderma fermentation source of β-glucanase (BG2). Each enzyme was added at 250 β-glucanase U/g (pH 6.8) feed. Addition of BG1 and BG2 improved weight gain by 28 and 28% respectively (p<0.0001) and FCE by 23% and 18% respectively (p<0.0001). Viscosity in the gizzard, first and last half of the small intestine was 10, 88 and 270 cp respectively in control broilers, an unacceptably high set of values, particularly in the hind-gut. Addition of either enzyme source significantly reduced viscosity in all gut sections and prevented the increase in viscosity noted in control fed birds. Foregut intestinal viscosity correlated well with weight gain (R2=0.907, p<0.0001) and FCE (R2=0.871, p<0.0001). The variance associated with weight gain and FCE was substantially reduced with enzyme addition, but particularly so with BG1. BG1 tended to reduce gizzard viscosity to a greater extent than BG2 suggesting that it is either more active at acid pH or that undefined associated activities (eg glucosidase) are necessary for optimum viscosity reduction.