The oral absorption of a 10-mg oral dose of the novel 5-hydroxytryptamine (5HT1D) agonist, 311C90, was compared during a moderate or severe migraine headache and in a migraine-free period in an open, two-period study. The safety and efficacy of 311C90 in acute migraine were also assessed. Twenty patients attended the clinics during a moderate or severe migraine attack and 18 patients returned for a second dose in a migraine-free period. 311C90 was less rapidly absorbed during a migraine attack compared to the migraine-free period, consistent with gastric stasis during a migraine attach. The median area under the curve (AUC) was 15.7 ng/mlh lower during a migraine (median AUC: 18.4 ng/ml.h, range: 0-60.8 ng/ml.h) compared to the migraine-free period (median AUC: 33.4 ng/ml.h, range 9.4-79.5 ng/ml.h) (95% confidence interval: 6.9, 25.3) and the time to reach maximum plasma concentration was delayed (n = 18). Eleven out of 20 patients experienced a significant improvement in migraine headache intensity at 2 h post-dose. Plasma 311C90 concentrations were generally higher in those patients who responded to treatment with 311C90 in the plasma, but there was one patient with no quantifiable 311C90 in the plasma whose headache improved. Minor adverse experiences were reported in 11 out of 20 patients during a migraine attack and in 11 out of 18 patients outside an attack. They occurred shortly following drug administration and were of short duration, but their occurrence did not appear to be related to plasma 311C90 concentration. There were no clinically significant changes in blood pressure or 12-lead ECG during the assessment period.