A　narrow-bore　capillary　(inner　diameter　=　2　mu　m)　zone　electrophoresis　method　was　developed　to　study　the　interaction　between　metformin　hydrochloride　and　bovine　serum　albumin.　Free　metformin　hydrochloride,　free　bovine　serum　albumin,　and　their　complex　were　completely　separated　in　a　volume　of　tens　of　picoliters.　The　association　constant　(K-a　=　1.04　x　10(3)　L　mol(-1))　and　the　number　of　binding　sites　(n　=　0.9789)　were　then　obtained.