We have previously shown that exogenously administered corticosterone affects muscarinic receptor binding in the chick embryo retina. Analysis with the selective antagonist pirenzepine has shown that both muscarinic receptor subclasses M1 and M2 are present in treated retinas. On the contrary, only M2 is detectable in controls. Moreover, receptor affinity for agonists is modified by hormone treatment. Since these studies did not show whether or not the hormone directly influences retinal cells, a similar study was performed on retinal tissue cultures. Cells were treated at day 5 in vitro for 24 hr with 1.10-* M corticosterone. Schatchard analysis of results obtained with 3H-quinuclidinyl benzilate (3H-QNB) binding showed no difference between treated and control cultures. However, displacement experiments demonstrated that both M1 and M2 were present in treated cultures, whereas controls had only M2. Also, receptor affinity for the agonist carbachol was modified, as already observed with in vivo studies. In addition, a new phenomenon was observed: treated cultures had a significantly higher number of cells. The possibility that the hormone can prevent cell death or stimulate cell mitosis is considered.