The contractile state of the sympathetically innervated superior and inferior tarsal muscles, also known as Mueller's muscles, was defined in 10 young adults by stimulating and then paralyzing these muscles pharmacologically in a randomized, double-masked study, using one eye for drug and the other for control. The drugs used topically were phcn-ylephrine hydrochloride 2.5 and 10% for muscle stimulation, and guanethidine 5% and thymoxamine hydrochloride 0.5% for muscle inhibition. A model is described for accurately examining and recording changes in the vertical palpebral fissure simultaneously bilaterally using magnified images from two high-resolution, high-sensitivity video cameras. The superior tarsal muscle averaged 50% of its total possible contraction when it was in its tonic state, and the inferior tarsal muscle averaged 67%. The drug instillations defined the mean maximum total excursion of the superior tarsal muscle as 3.0 mm, with 1.5 mm of upward displacement and 1.5 mm of downward displacement from its tonic position. The inferior tarsal muscle had a mean maximum total excursion of 0.3 mm, with 0.1 mm downward displacement and 0.2 mm upward displacement from its tonic position.