Objective: To observe the influences of infusing small volume of 7.5% NaCl solution (HS) in vivo,and the different volume ratio of plasm of rich platelets (PRP) to HS or sucrose in vitro on the function of platelet. Method:Twenty-nine patients undergoing middle or small operation were divided randomly into HS group and normal saline group(NS). Blood samples were taken before infusion and,5,30,60 and 120 min after infusion,to measure hematocrit (HCT),platelets (PLT) count, maximum aggression rate (MAR),thromboxance B_2(TXB_2),granule membrane protein-140(GMP-140) and platelet factor 3 activety (PF_(3a)T). In addition,fifteen blood samples of blood donors were taken,and divided control group,HS group,sucrose group and NS group. The volume ratio in each group were arranged in 1/25,2/25,4/25 and 8/25. Osmotic concentration of 1/25 was equivalent to that induced with small volume hypertonic saline infusion. Result: Compare with NS group,the HCT and PLT count were decreased after infusion of small volume of HS. When the volume ratio ≥2/25, all measure values of the MAR, TXB_2, GMP-140 and PF_(3a)T were decreased in vitro,and profoundly inhibited following osmolarity increase of HS. Conclusion;Small volume of HS may be safely used,but severe inhibition of functions of platelet may occur when the infusing volume of HS exceeded more than twice of small volume of HS.