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.