The platinum tip of a scanning tunneling microscope is employed to direct the electropolymerization of aniline on nanometer-scale regions of a graphite surface which is immersed in an aqueous, aniline-containing electrolyte. The electropolymerization of aniline is initiated by two bias voltage pulses which are applied in rapid succession : The first-(tip-negative) 6.0 V x 5 mu s-causes the formation of a pit in the graphite surface, and the second-(tip-negative) 3.0 V x 50 mu s-induces the oxidation and the subsequent polymerization of aniline at this pit to form a particle of polyaniline (PANI). These PANI particles varied from 100 to 600 Angstrom in diameter and 10 to 200 Angstrom in height. Nanoscopic PANI particles prepared by this method are shown to be electrochemically responsive : A comparison of in-situ STM images of individual PANI particles acquired at sample potentials which are positive and negative of the PANI oxidation potential reveals that the volume of a particle is larger by approximate to 30% in the oxidized emeraldine form than in the reduced leucoemeraldine form as a consequence of the higher solvent and anion content of the oxidized PANI particle. The results of in-situ STM experiments, ex-situ electrochemical measurements, and Monte Carlo simulations of transport collectively provide information on the mechanism of PANI particle growth. On the basis of these data, it is postulated that the aniline involved in PANI particle synthesis is derived from an adsorbed monolayer at the graphite surface.