Muscarinic receptor signalling has been implicated in both the embryonic and postnatal development of ocular structures as well as in myopic eye growth. A radioligand binding assay was used to determine whether changes in muscarinic receptor density and/or affinity occurred in the chick retina, choroid and sclera during early post-hatching development or with the induction of myopia. Specific receptor binding sites were saturable with increasing concentrations of the muscarinic receptor ligand [3H]N-methyl-scopolamine in the retina and choroid but not in the sclera. In normal eyes, binding density in the retina was not altered from age P5 to P10 (447+/-14 vs. 411+/-13 fmol/mg of protein, P=0.07). However, in the choroid, the number of receptor binding sites significantly increased between P5 and P10 (637+/-39 vs. 1125+/-121 fmol/mg of protein, P<0.01). Binding affinity (K(D)) was not altered with age in either the retina or choroid. Myopia was induced in chicks by deprivation of form vision, using translucent diffusers, from age P3. Despite the induction of significant degrees of ocular elongation and myopia at P5 (-8.7+/-0.3 D, P<0.01) and P10 (-22.5+/-1.3 D, P<0.01), neither muscarinic receptor density nor affinity were altered in the retina or choroid of myopic eyes. These findings indicate that regulation of muscarinic receptor numbers in the chick choroid is occurring in normal post-hatching development of this tissue. However, myopic eye enlargement was not associated with changes in muscarinic receptor protein expression in the chick retina and choroid.