Chyme concentrations and total recoveries of furazolidone (5 mg/kg body-weight) were determined by a HPLC-method, after oral administration of two different furazolidone formulations to piglets (n = 6) and pre-ruminant calves (n = 8), provided with an ileal re-entrant canula. Additional blood samples were taken from the calves to measure the time dependent plasma levels of furazolidone. In the case of the normal crystalline preparation, the results indicate an almost complete absorption of the drug from the upper parts of the digestive tract. In both species, 96-99% of the dose had been absorbed by the time it reached the end of the ileum. The mean ileal recovery of the newly developed furazolidone formulation in calves and piglets was 14% and 38%, respectively. In calves the observed maximum plasma concentrations of furazolidone after oral application of the sustained release formulation were 14 times lower than with the normal crystalline preparation.