In the article a model of histamine kinetics is described. A motivation of this project was to investigate the hypothesis that methylhistamine may be a marker of histamine appearance in plasma. A model has been made to support the hypothesis. Since metabolic and transport pathways of histamine and methylhistamine are complex and not very well known, the relationship between histamine and methylhistamine should be elucidated by mathematical modelling. From experimental data and the information in the literature, a nonlinear and time-varying four-compartment model is proposed. Extensive release of histamine from mast cells when methylhistamine is injected, is modelled as histamine to methylhistamine ratio control loop.