Abstract Platelets stored at 4 ° develop morphological changes which precede decreases in in vitro aggregation and recovery from hypotonic shock. Platelets maintained at 4 ° for 60 min lack microtubules and lose their discoid shape as a result of the formation of pseudopodia. Exposure to continuous cold for 24 hr causes irreversible platelet shape changes and induces spontaneous aggregation. Quantitative electron microscopic analysis of platelet microtubules showed that platelets lost their ability to reassemble microtubules after 24 hr of continuous storage at 4 °. However, interruption of continuous cold storage with short, repeated pulses of 37 ° (temperature cycling) extended microtubule reassembly capabilities for 72 hr and maintained platelet shape change capabilities. A comparison of platelets stored continuously at 4 ° with those stored by temperature cycling showed that temperature-cycled platelets possessed improved aggregation responses and significantly improved recoveries from hypotonic shock. These data indicate that 37 ° reactivation pulses during cold storage provide platelets with markedly improved morphology and in vitro test scores and suggest that temperature cycling may be used to extend blood bank storage of platelets.