Poikilothermic animals respond to chronic cold by increasing phosphoglyceride unsaturation to restore the fluidity of cold-rigidified membranes. Despite the importance of this compensatory response, the enzymes involved have not been clearly identified, and the mechanisms that control their activity are unknown. In carp liver, cold induces an 8- to 10-fold increase in specific activity of the microsomal stearoyl coenzyme A desaturase. Cold-induced up-regulation of gene transcription resulted in a 10-fold increase in desaturase transcript amounts after 48 to 60 hours. However, this increase was preceded by the activation of latent desaturase, probably by a posttranslational mechanism. These two mechanisms may act sequentially to match desaturase expression to the demands imposed by a progressive decrease in temperature.