To investigate the mechanisms of action of metformin, insulin receptor binding and the activity of several insulin-controlled metabolic pathways were measured in adipocytes taken from 10 obese Type 2 diabetic patients treated for 4 weeks with either metformin (0.5 g x 3 daily) or matching placebo using a double-blind crossover design. Metformin therapy was associated with a significant fall in serum fructosamine levels (3.1 +/- 0.4 vs 2.8 +/- 0.4 mmol l-1, p less than 0.02) as well as fasting (10.8 +/- 2.4 vs 9.4 +/- 2.1 mmol l-1) and daytime (11.5 +/- 2.4 vs 10.0 +/- 2.2 mmol l-1) plasma glucose concentrations (p less than 0.05). Fasting and postprandial plasma levels of C-peptide and insulin were unchanged. While fasting plasma lactate concentrations remained unaltered after metformin, a rise was noted in response to meals (from 1.4 +/- 0.1 to 1.8 +/- 0.2 mmol l-1, p less than 0.05). Adipocyte insulin receptor binding was unaffected by drug treatment. Moreover, no insulin-like effects or post-binding potentiation of insulin action could be found on adipocyte glucose transport, glucose oxidation, lipogenesis, glycolysis or antilipolysis. A complementary in vitro study using adipocytes from non-obese healthy volunteers failed to show any direct effect of metformin on adipocyte insulin binding or glucose transport and metabolism, at media drug concentrations corresponding to therapeutic plasma levels.