Previous studies have demonstrated that muscarinic antagonists, such as atropine and pirenzepine, block form deprivation myopia in avian and mammalian models. The aim of the present investigation was to establish dose-response curves for intravitreal and subconjunctivally injected pirenzepine and to determine receptor specificity. Chicks were monocularly deprived of form vision for five days and received daily injections of either pirenzepine or saline. Keratometry, retinoscopy and A-scan ultrasonography of axial ocular dimensions were then taken. Intravitreally injected pirenzepine was effective at preventing form deprivation myopia in a dose dependent manner with an ED50 of 175 micrograms. A 500 micrograms dose totally prevented induced myopia (+0.9 D versus -13.7 D) and axial enlargement (-0.14 mm versus +0.32 mm). Daily subconjunctival injection of pirenzepine was significantly less effective in preventing form deprivation myopia. Form deprivation myopia could still be induced in animals which had undergone pirenzepine treatment. Pirenzepine was effective in preventing the axial elongation associated with experimental myopia in a dose dependent manner and via a functional not toxic mechanism.