Four compounds of therapeutic value in the treatment of migraine attacks or for the prophylaxis of migraine and other vascular headaches, ergotamine, dihydroergotamine, methysergide and pizotifen are given in low dosages and achieve low in vivo concentration. Human plasma levels were determined by measurement of radioactivity after oral administration of the corresponding radioactively labeled compounds. The labeling of the four compounds with tritium or carbon-14 is briefly described. The procedure to measure radioactivity of the blood samples is given. The values from single dose experiments in healthy volunteers were analysed based on a compartment model which allows for the formation of metabolites. The calculated pharmacokinetic constants have been used for simulations and predictions of plasma levels after different therapeutic dosages.