Characteristic responses of motor neurons to injury include an apparent increase in the phosphorylation of C-terminal domains of neurofilament proteins in the perikaryal and dendritic compartments. This change was induced in dissociated cultures of embryonic spinal cord by activation of protein kinase C (PKC). PKC was activated by: (a) exposure of cultures to 10 nM 12-o-tetradecanoyl phorbol 13-acetate (TPA); (b) microinjection of 1 mM dioctanoylglycerol (diC8) directly into perikaryal of motor neurons;(c) addition of 10 microM diC8 to culture medium. Activation of PKC led to different immediate and long term effects on neurofilaments of motor neurons. After 30 minutes (min), fragmentation of the neurofilament network was observed by labeling with antibodies to low and high molecular weight neurofilaments proteins; glial filaments were disassembled after 10 min and reassembled by 1 hour (h). From 4 to 24 h, motor neuron were observed with extensions of perikaryal cytoplasm or massive enlargements of proximal dendritic processes, both containing intact neurofilament networks. Over 1 to 12 days, there was a gradual increase in the number of motor neuronal perikarya immunoreactive with antibodies to neurofilament proteins phosphorylated at KSP sites on the C-terminal domains (SMI31, SMI34). It is proposed that activation of PKc secondary to other injurious events may contribute to the changes in neurofilaments observed in motor neuron diseases.