The bioavailability of two galenic formulations of 5-methoxypsoralen (5-MOP) is described. A suspension in soft gelatine capsules was tested against a micronized powder in hard gelatine capsules on six volunteers, both in a dosage of 40 mg. The comparison of the AUC shows a significant better availability of the suspension (p less than or equal to 0.05). The detection limit of 5-MOP is 2 ng/ml plasma. The described analytical method is accurate and rapid--recovery near 100%--and is suitable for routine analysis. The pharmacokinetic parameters for 5-MOP are 2.3 h for the elimination half-life and 0.344 h-1 for the elimination rate constant.