The synthesis of noradrenaline (NA) was studied in vivo in the hypothalamus, the brain stem and the "rest of brain" of rats which were exposed to 4 degrees C for 0.5, 2.5 or 24 h. The rate of NA synthesis was estimated 30 min after an i.v. injection of 3H tyrosine (TY) by evaluation of the ratio: 3H-NA specific activity/3H-TY specific activity. Cold exposure did not have the same effect on NA synthesis in the three brain areas. In the hypothalamus, the rate of NA synthesis was increased by a factor of 1.7 and 2 after 0.5 and 2.5 h of cold exposure, respectively and returned to control values after 24 h of cold exposure. In the brain stem, NA synthesis was enhanced by a factor of 1.5 at 2.5 h and returned also to control values at 24 h. In the "rest of brain", cold exposure did not alter NA synthesis at the three intervals studied. These results indicate that the activation of central noradrenergic neurones by cold is only a transient response which is restricted to specific brain areas and which can be interpreted as the consequence of an initial general stress associated with cold exposure.