Abstract1) Fast axoplasmic transport in mammalian nervein vitrowas studied using an isotope labeling technique. The rate of outflow in cat sciatic nerve fibers of 410 mm/dayin vitrowas reduced at temperatures below 38°C with aQ10of 2.0 in the range 38–18°C and aQ10of 2.3 at 38–13°C.2) At a temperature of 11°C a partial failure of transport occurred. At temperatures below 11°C a complete block of fast axoplasmic transport occurred, a phenomenon termed “cold‐block.” No transport at all was seen over the temperature range of 10–0°C for times lasting up to 48 hr.3) Transport was resumed after a period of cold‐block lasting up to 22 hr when the nerves were brought back to a temperature of 38°C. Some deleterious effects due to cold‐block were seen in the recovery phase as indicated by a reduction in crest amplitude, change in its form, and slowed rate.4) The ∼P level (combined ATP and creatine phosphate) remained near control level in nerves kept at low or cold‐block temperatures for times as long as 64 hr. The reduction in fast axoplasmic transport rate seen at low temperatures for times up to 22 hr was therefore considered due to a decrease in the utilization of ATP, a concept in accord with the “transport filament” model proposed to account for fast axoplasmic transport.5) The sloping of the front of the crest over the temperature range of 18–13°C suggests an additonal factor at the lower temperatures. A disassembly of microtubules is discussed as a possible explana