The tolerability of 311C90, a novel, selective and highly effective 5-HT1D receptor agonist in development for the acute treatment of migraine, has been evaluated in a number of clinical pharmacology and patient studies across the dose range 1-50 mg. 311C90 has been well tolerated across the entire dose range and no clinically relevant changes in routine laboratory parameters, blood pressure or ECG recordings have been observed. Adverse experiences reported are generally dose related, mild to moderate and resolve spontaneously. Chest-related symptoms occur infrequently and the cardiovascular safety profile of 311C90 is considered particularly favourable. 311C90, therefore, possesses a desirable safety profile which is well suited to broad-based outpatient administration.