Increased synaptic stimulation of sympathetic neurons in vivo causes a delayed increase in the activity and the amount of tyrosine hydroxylase (TH). To determine whether these changes result from an increase in the messenger RNA for TH, the rat preganglionic cervical sympathetic trunk was electrically stimulated unilaterally for 90 min, and 48 h later RNA was extracted from stimulated and contralateral control superior cervical ganglia. Northern blots probed with a cDNA for TH demonstrated that nerve stimulation produced about a 2.5-fold increase in the amount of TH mRNA in the ganglion. These results indicate that synaptic stimulation leads to an increase in TH mRNA, either by increasing the rate of transcription of the TH gene or by increasing the stability of its mRNA.