Using primed-continuous 3-³H-glucose infusion, basal glucose production rate has been reported to be 140% higher than normal or almost normal in hyperglycaemic patients with Type 2 (non-insulin-dependent) diabetes mellitus. To determine whether these markedly different results could be due to the mode of priming: fixed or adjusted, or the mode of calculation: steady state or non-steady state equations, we studied 11 patients with Type 2 diabetes (fasting plasma glucose 8–20 mmol/l). For 6 h 3-³H-glucose (0.40 μCi/min) was infused preceded by a priming dose of 40 μCi (fixed priming), or 40 μCi · plasma glucose (mmol/l)·5⁻¹ (adjusted priming). In diabetic patients the plasma glucose concentration was not constant but declined 0.52±0.07 mmol·l⁻¹· h⁻¹. Furthermore, the rate of fall was correlated to the fasting plasma glucose concentration (r=0.90, p<0.01). Thus, the fasting state was not a steady state condition. Using adjusted priming a constant tracer steady state level was obtained within 60 min. In contrast, using fixed priming tracer steady state was not reached within 6 h. The initial tracer level was far below, and increased in time towards the steady state level observed after adjusted priming. Consequently, using Steele's equations after fixed priming, glucose production rates calculated after 90–120 min were overestimated in proportion to fasting hyperglycaemia. In conclusion: The fasting state in patients with Type 2 diabetes is not a steady state condition. Adjusted priming seems most appropriate in Type 2 diabetes. By estimating glucose production 2 h after fixed priming or assuming steady state conditions, most previous studies may have overestimated basal glucose production in Type 2 diabetes in proportion to fasting hyperglycaemia.