The study was done in healthy volunteers to investigate the vasoconstrictor activity of three commercially available formulations of fluocinolone acetonide (Synalar gel, cream and ointment) and to determine whether they were bioequivalent. The influence of dilution of the cream formulation on activity was also examined. Twenty mg of each topical formulation was applied, followed by occlusion for 6 h, and the potency was evaluated by measuring the vasoconstrictor effect using tristimulus colour analysis. The cream formulation was more potent than the ointment, with the gel having intermediate strength. Dilution did not significantly reduce activity. While choice of an appropriate topical corticosteroid formulation largely depends on the nature of the lesions, this study has shown that different formulations of nominally the same concentration have different potencies; dilution up to 1 in 10 produced no significant reduction in potency. The potency classification in the British National Formulary for formulations of Synalar may need revision.