A toxicokinetic (TK) model was developed to describe the inhalation exposure in humans to methyl formate (MF), a catalyst used in foundries, and to discuss biological monitoring. The TK model consisted of four compartments: MF, the metabolites--methanol (MeOH) and formic acid (FA)--and, in addition, a urinary compartment describing the saturable reabsorption of FA. Levels of MeOH and FA in urine, from an experimental study (100 ppm MF, 8 h at rest), validated the present model. The TK model describes well the general behaviour of MeOH and FA in urine after MF exposure. A nonlinear and a linear relationship respectively, was predicted between MF exposure and FA or MeOH excretion in urine, and this has previously been seen after occupational MF exposure. The present model has been modified to simulate MeOH exposure as well. Generally low exposures (concentration or exercise) produce only marginal increases in FA urinary excretions, but when exposure is elevated, urinary FA excretion increases because of saturation in the mechanism of reabsorption. Using FA urinary excretion as the critical indicator, because of its link to health effects, an occupational exposure limit value for MF of no greater than 50 ppm should be selected (based on predictions with the TK model). MeOH in urine can be considered as a biomarker for MF at low exposure, because of lower background values and of a linear relationship with exposure. At higher exposures, however, FA could be used as a biomarker as it becomes progressively more sensitive. But the use of biological monitoring for MF is difficult because of individual variations in background values. Under the present state of knowledge both FA and MeOH should be used to estimate only group exposures, rather than individual exposures.