Heparin is a mucopolysaccharide from mammalian tissues. It is an anticoagulant drug used universally. Of naturally occurring mucopolysaccharides only heparin competitively inhibit Ribonuclease (RNase) in its physiological range. Because of high bleeding risks due to the continuous consumption of heparin its determination in blood is critical. Many assays have been devised to measure this substance in blood. This paper describes manual and flow injection methods for the determination of heparin by the inhibition of ribonuclease. Manual spectrophotometric method was found very simple and sensitive. By ihis method heparin can be determined in its physiological range. Calibration graph was linear in the range 1.0-16 llg mI·'. The limit of detection was 1.0 llg mI·'. However, sensitivity decreased in flow injection method while using immobilised Rnase.