The effect of temperature changes on the number of microtubules in non-myelinated axons has been studied in cat inferior mesenteric ganglion/hypogastric nerve preparations incubated at various temperatures in Eagles minimal essential tissue culture medium in vitro. At 37 degrees C the non-myelinated axons contained 28.4 plus or minus 0.8 S.E.M. (54) microtubules per axon. After incubation at 0 degrees C for 4 h this number fell to 2.3 plus or minus 0.1 S.E.M. (41) but returned to normal levels when the nerves were rewarmed. This loss of microtubules on cooling the nerves and their reappearance on rewarming was a rapid process; it was independent of the influence of the nueronal cell body and of protein synthesis within the axon. The preservation of the microtubules was improved when D2O was present in the incubation medium. Reformed microtubules appeared to function normally with respect to their possible role in the transport of noradrenaline storage vesicles along the axons.