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The fixation of the neurotransmitter dopamine in the central nervous system by perfusion with formalin solutions seems to take place mainly via the formalin-induced condensation product norsalsolinol. In the present investigation the influence of microwave irradiation of the formalin-induced condensation of dopamine was studiedin vitroandin vivoby making use of different, relatively low, formalin concentrations. It appeared thatin vitroandin vivothe dopamine conversion was complete with 4% formalin and no influence of microwaves was noted. However, by making use of much lower formalin concentrations (0.2% and 0.4%) the condensation of dopamine was strongly augmented,in vitro(200%) andin vivo(at least 500%) using microwave techniques. There was a considerable loss in non-microwaved tissue (30%) after perfusionin vivo. This was lower (10%) in microwaved tissue. In experiments with perfused brain tissue which allowed a more complete calculation, a loss was found. This might be caused by a strong binding of dopamine and/or norsalsolinol to tissue components or to side reactions that could not be traced by the present experimental techniques.