The introduction of heavy metals directly into living neuronal tissue has received little attention as a method for defining connections in the vertebrate nervous system. Procedures are described for the use of cobaltous-lysine to trace visual pathways with light microscopy. The success of the technique depends on a number of variables including: the method by which the tracer is applied, the survival period, and the method of intensification. Best results were obtained by soaking the optic nerve of living animals in the filling solution for 24 hr and by using a modification of the Timm's silver sulfide intensification procedure. Several factors that lead to poor results are discussed. Cobaltous-lysine is capable of both anterograde and retrograde labeling and can fill very sparse projections. Furthermore, the success of the fill can, at times, be evaluated prior to histology.