Although heparin was reported in the 1960s to inhibit renin activity, this has not always been confirmed by other investigators. Hence, we re-examined whether heparin really inhibits renin or not. Renin activities were determined by radioimmunoassay of angiotensin I generated at pH 7.4. (i) No significant difference was found between the two kinds of plasma samples obtained with heparin and with EDTA as anticoagulant, in ARC (renin activity with addition of sheep renin substrate), TRC (ARC after activation of inactive renin by trypsin), or PRA (plasma renin activity without additional substrate). (ii) Even in higher concentrations of heparin up to 500 U/mL, neither PRA, ARC, nor TRC of plasma was affected significantly. (iii) Heparin, in concentrations up to 500 U/mL, exerted no significant effect on TRC of the media of human vascular smooth muscle cell culture. In conclusion, heparin does not exert any significant inhibitory effect on human renin nor does it affect activation of inactive renin by trypsin in the range of concentration of practical use, under the conditions employed in this study.