Leaves from the tropical tree legume calliandra (Calliandra calothyrsus) were harvested and fed either fresh frozen (F) or dried (D) as a supplement (200 g dry matter (DM)) to sheep (n=4) given barley straw ad libitum in metabolism cages in a controlled-environment animal house. Sheep given these diets were intra-ruminally infused (0·5 litres/day) with either water or a solution containing 100 g/l polyethylene glycol (PEG). These treatments were administered in a randomized complete block design, and the results statistically analysed. Drying significantly increased the cell wall (cellulose, hemicellulose) and lignin contents, but decreased the condensed tannin (butanol-HCl method) content (F=35·7, D=21·7 g/kg DM). Both drying and PEG infusion significantly (P<0·05) increased straw organic matter (OM) intake, but the effects of drying plus PEG were not additive. Organic matter digestibility of the diets was significantly (P<0·05) increased by drying (F=433 g/kg, D=486 g/kg), but not by the infusion of PEG. Total nitrogen (N) digestibility was significantly (P<0·05) increased by drying (F=207 g/kg, D=316 g/kg) in the absence of PEG, and by PEG for both frozen (−PEG=207 g/kg, +PEG=392 g/kg) and dried (−PEG=316 g/kg, +PEG=434 g/kg) calliandra. PEG infusion significantly increased the proportion of feed N degraded in the rumen (−PEG=0·41, +PEG=0·52) and rumen ammonia concentrations (−PEG=63, +PEG=103 mgN/l), but had no significant effect on the rate or efficiency of microbial protein synthesis in the rumen. PEG infusions significantly increased the amount and proportion of N intake absorbed from the lower digestive tract, but had no significant effect on overall N balance. Drying decreased the tannin losses (as % intake) occurring in the rumen (F=37·3%, D=24·2%), and PEG infusion increased these losses for both frozen (−PEG=37·3%, +PEG=72·5%) and dried (−PEG=24·2%, +PEG=68·7%) calliandra supplements. It was concluded from these studies that the presence of tannins in fresh (frozen) calliandra depressed feed utilization, and that drying was an effective means of improving the nutritive value of calliandra when fed as a supplement to low quality straws.