Nerve fiber outgrowth and elongation were studied i n spinal ganglia explants cultured with NGF. The effects of cold and colchicine treatments on the degradation of microtubules and on the outgrowth and elongation of nerve fibers were determined. Colchicine inhibited nerve fiber outgrowth and elongation, and degraded the microtubules of the nerve cells, producing a n abundance of 100 A filaments. Cold treatment inhibited fiber outgrowth and elongation, but did not induce degradation of the microtubules of the nerve fibers. The effect of cold treatment on the growth of nerve fibers was reversible. The microtubules of these nerve fibers therefore exhibited differential stability i n reaction to microtubule degradative agents, in that they were degraded by colchicine and were resistant to cold treatment. The results indicate also that microtubules may be involved in the outgrowth and elongation of nerve fibers. However, since nerve fibers exposed to colchicine did not retract, it is suggested that microtubules may not be solely responsible for the maintenance of the fiber processes of these cells, but that contacts with other cells and with a supporting matrix could be important in the maintenance of the asymmetric nerve cell processes.