This paper applies a simple analysis of the: statistical noise inherent in sensing concentrations of diffusible chemical factors to the problem of guidance of developing axons. We show that growth cones may be able to detect chemical changes of 0.5% across their width, and that guidance by a gradient is limited to distances below about 1 cm, even for a gradient of ideal shape. (C) 1999 Elsevier Science B.V. All rights reserved.