Developing spinal cords of birds and mammals exhibit rhythmic waves of spontaneous electrical activity during initial motor axon outgrowth. In chick embryos electroporated channelrhodopsin-2 can be activated in-ovo to chronically drive waves at different frequencies, avoiding the complication of altering frequency by drugs that affect different neurotransmitter systems, as carried out in previous experiments. Motor axon pathfinding was found to be highly sensitive to the frequency of such waves, as modest decreases in frequency selectively affected the binary dorsal-ventral pathfinding decision and the expression of molecules involved in this guidance choice, while modest increases selectively disrupted the subsequent motoneuron pool-specific pathfinding choice. The ability to precisely control in-vivo wave frequency non-invasively by light should facilitate the elucidation of the intracellular signaling pathways underlying these divergent pathfinding choices and the extent to which the frequency versus the pattern of the waves is important. This approach should also aid in the discovery of other activity dependent aspects of spinal cord development. In the retina and spinal cord waves trigger Ca2+ transients which in turn generate transients of cAMP. To explore the role of cAMP transients we have used a light activated bacterial adenylyl cyclase to cause cAMP transients in cultured motoneurons and in intact spinal cords in-ovo. Such transients affected growth cone behavior of cultured motoneurons and generated spontaneous waves in isolated spinal cords and in intact embryos in-ovo. While additional experiments are needed these optogenetic tools should prove valuable in further characterizing the roles of Ca2+ and cAMP transients and their possible interactions in motor axon pathfinding and circuit development.