Crossbred wether sheep, each fitted with a cannula in the rumen, abomasum and terminal ileum, were given a concentrate diet either untreated or treated with formaldehyde. A bolus of digesta containing two soluble markers, one of which remained in solution ( 51Cr-EDTA ) , while the other was adsorbed onto particulate matter (103Ru-P), was introduced into each sheep: first into the terminal ileum, then into the abomasum and finally into the rumen. After each dose, faeces were collected at short intervals until all marker had been excreted and mean retention times were calculated from the time course of marker excretion. 103Ru-P was retained much longer than 51Cr-EDTA in the stomach; this was due mainly to retention in the rumen, but 103Ru-P was also retained longer in the abomasum. The mean retention times of 103Ru-P and 51Cr-EDTA in the hind gut did not differ. Formaldehyde treatment of the diet decreased mean retention times in the rumen and in the hind gut, but increased them in the abomasum and small intestine. The markers were fully recovered; 103Ru-P was found entirely in the faeces whereas 4.9% of the dose of 51Cr-EDTA was absorbed and appeared in the urine. The absorption of 51Cr-EDTA from different sections of the gastrointestinal tract appeared to be related to sectional mean retention time.