The plasma level of the GH-independent insulin-like growth factor binding-protein-1 (IGFBP-1) is regulated inversely by insulin. In this study the effect of insulin and changes in the glucose concentration on in-vitro IGFBP-1 secretion by the Hep G2 cell line was studied. Media from confluent cells in 12 replicates were collected for consecutive periods: initial control (20 h), study (6 h) and recovery (20 h). Insulin suppressed IGFBP-1 secretion maximally at 100 mU/l (-32%) within 6 h. The secretion of IGFBP-1 was stimulated by a decrease in the glucose concentration in the medium, maximally (+25%) with a decrease from 24 to 6 mmol/l. Stimulation by varying glucose levels and suppression by insulin of IGFBP-1 secretion persisted on return to control conditions after the removal of physiological concentrations of glucose (4-12 mmol/l) and insulin (50-500 mU/l). The findings in the Hep G2 cell line that a variation in the physiological concentrations of glucose and insulin each independently regulate IGFBP-1 secretion suggest that this cell line may be a suitable model for further in-vitro studies of the regulation of secretion of IGFBP-1.