Bovine serum albumin greatly enhanced the cholinergic response mediated by neuronal nicotinic acetylcholine receptors in chick ciliary ganglion neurons. The enhancement exceeded 5-fold in some experiments (mean +/- standard error, 3.26 +/- 0.43-fold) and was rapid, was dose dependent, and occurred without changes in the unitary conductance or the mean open time of the acetylcholine receptor channel. This lack of detectable change in permeation or kinetic properties suggests that bovine serum albumin might increase acetylcholine responses by increasing the number of functional receptors. The enhancement appears to be specific to the albumin molecule, because activity could not be removed by detergent extraction, gel filtration, or dialysis. Acetylcholine responses in these cells are known to be enhanced by a cAMP-dependent mechanism that converts existing acetylcholine receptors from a nonfunctional to a functional state. We found that the enhancement by bovine serum albumin occurred without an increase in cAMP and that pretreatment with membrane-permeable cAMP analogs prevented any additional enhancement of the cholinergic response by bovine serum albumin. These observations are consistent with a cAMP-dependent modulation of the enhancement produced by bovine serum albumin or a convergence of the two enhancement mechanisms onto a single pathway.