A quadrupole ion-trap mass spectrometer having half the dimensions of the commercial ion-trap instrument has been built. When operated in the mass-selective instability mode using the commercial electronics and data system and the standard vacuum system, it yields a mass range of > 2500 Da. This represents achievement of the expected fourfold increase in mass range upon halving the trap radius (to 0.5 cm). The sensitivity of the miniature trap is comparable to that of the commercial ion-trap; however, resolution is inferior, partly due to the high mass-scan rate. The effects of using relatively high pressures of helium as a buffer gas are discussed in relation to improving sensitivity and resolution. The use of the high-mass ion-trap as a device for studying biomolecules is discussed and methods of extending the mass range even further are suggested.