The linearity of the pharmacokinetics of the metamizol metabolites 4-methyl-amino-antipyrine (4-MAA), 4-amino-antipyrine (4-AA), 4-formyl-aminoantipyrine (4-FAA), and 4-acetyl-amino-antipyrine (4-AcAA) has been studied after administration to 15 healthy male volunteers of single oral doses of 750, 1500, and 3000 mg metamizol. The trial was open, randomized, and cross-over, with a one-week interval between dosing days. Metabolite concentrations in serum and urine were measured using reverse-phase HPLC. The mean Cmax of 4-MAA increased linearly with dose whereas its AUC was not proportional to dose after administration of 1500 and 3000 mg. With 4-AA, the increase in mean Cmax was linear, but the increase in AUC was not. The increases in mean Cmax and AUC for 4-FAA after doses of 1500 and 3000 mg were not proportional to the dose. The increases in mean Cmax and AUC for 4-AcAA were roughly proportional to the increase in dose. There were no significant differences in renal clearance between doses for any of the four metabolites. The observed non-linearities reflect the saturability of metabolic pathways. However, although they were statistically significant, the deviations from linearity were marginal and should not be of clinical relevance to the analgesic efficacy of metamizol in the dose range tested.