The synthesis of catecholamines (CA) has been studied in the heart, spleen, submaxillary glands and adrenals of rats exposed to 4 degrees C for 2.5, 24 or 48 h. The synthesis rate has been estimated 30 min after an i.v. injection of 3H tyrosine (TY) by the evaluation of the ratio: 3H-CA specific activity/3H-TY specific activity. In the sub-maxillary glands, cold exposure reduced the noradrenaline (NA) synthesis by 40% at times 24 and 48 h. In the spleen, NA synthesis was multiplied by a factor 1.6 at times 2.5 and 24 h and 2.8 at time 48 h. In the heart, it was increased by a factor 1.3 after 2.5 h, 2.8 after 24 h and 5.5 after 48 h: an important fall in cardiac NA level was observed during the first 24 h of cold exposure indicating that the synthesis capability was unsufficient to compensate the cold-induced NA release. In the adrenals, adrenaline + NA synthesis was not significantly enhanced during the first 24 h of cold exposure and increased by a factor 2.4 at time 48 h. The important increases in CA synthesis which are observed during the 24-48 h interval are likely consecutive to the induction of tyrosine hydroxylase which has been reported in the rat exposed to cold.