The success of the M(1)-selective muscarinic antagonist pirenzepine in preventing myopia development in animal models implicates a role for the M(1) receptor. However, the relatively high dose of pirenzepine required may indicate that the drug acts through another receptor subtype. This study examined whether the M(4)-selective antagonist, himbacine, could also prevent myopia. Daily intravitreal injections of himbacine inhibited the inducement of myopia in chick eyes in a dose- dependent manner. Doses < or = 200 microg caused no significant inhibition of induced myopia compared to controls (-13.7 +/- 2.3 vs -16.2 +/- 0.9D, ANOVA p = 0.37), whilst a dose of 800 microg almost completely inhibited the induced myopia (-2.4 +/- 2.0, p < 0.01). Findings demonstrate himbacine is effective at preventing the development of myopia in chick and implicates a role for the M4 receptor.