The use of permeable cell culture inserts (porous membrane filters, culture cups, and culture wells) has become widespread in recent years. The numerous advantages of this application have revolutionized in vitro cell biology and it is now used extensively in electrophysiological studies of epithelium. The U9521 Ussing chamber is designed to hold tissue culture inserts using low-cost adapters. The chamber is mounted in the standard support stand and procedures for experimentation are the same as with the "classic" Ussing system. This is a very versatile and economical system for using tissue culture inserts in transepithelial studies. Adapters in a large variety of sizes for most brands of culture inserts are available.