L-Arginine analogs are generally used as inhibitors of nitric oxide (NO) synthesis in a variety of tissues. We studied the effects of two analogs of L-arginine on L-arginine transport and NO synthase activity in human polymorphonuclear leukocytes (PMN) and platelets. Both NG-nitro-L-arginine methylester and N omega-nitro-L-arginine reduced the uptake of 3H-L-arginine in human PMN and platelets in a concentration-dependent fashion. The inhibitory effect of these analogs on the uptake of 3H-L-arginine was greater in PMN than in platelets (P < .05). Both agents also modestly inhibited NO synthase activity in platelet cytosol, whereas neither had significant and specific effect on NO synthase activity in the PMN cytosol. NO synthase activity in intact PMN, but not in platelets, was insensitive to exogenous Ca++. Reverse transcription-polymerase chain reaction products showed the presence of endothelial constitutive (756 bp) and the neuronal constitutive (629 bp) NO synthase isoforms in human platelets and PMN, respectively. This was further confirmed by Southern analysis. Thus the classic inhibitors of NO synthesis primarily decrease L-arginine uptake in PMN and platelets, and significantly affect NO synthase activity only in platelets. These differences may, at least in part, be due to the presence of different NO synthase isoform in platelets and PMN.