Cells from a line of transformed quail fibroblasts (QT-6) were transfected with cDNAs coding for subunits of the mouse muscle nicotinic ACh receptor (AChR). Stable clones were selected that expressed subunits of the fetal-type AChR (alpha, beta, gamma, delta) or the adult-type AChR (alpha, beta, epsilon, delta). The receptors had the appropriate burst durations and single-channel conductances for the fetal or adult type, respectively. Each type of receptor had a dose- response relationship that was close to a square law at low concentrations of ACh, implying that they contained two ACh-binding subunits. The metabolic stability of surface fetal and adult receptors was identical (about 10 hr half-life), for two independent clones expressing fetal and two expressing adult AChR. The metabolic stability was unaffected by treatment with okadaic acid, which enhanced receptor phosphorylation. d-Tubocurarine (dTC) blocked both the binding of alpha- bungarotoxin (BTX) to the cells and the ACh-elicited current. dTC blocked BTX binding with indistinguishable efficacy for both fetal and adult AChR. However, it was sixfold less effective at blocking ACh- elicited current from fetal AChR. At least part of the difference results from the ability of fetal receptor channels to open when the receptor has one ACh and one dTC molecule bound, whereas channels of heteroliganded adult receptors do not open. The data indicate that the subunit composition directly affects physiological and pharmacological properties of muscle AChR, but has little effect by itself on metabolic stability.