1. Isometric tension was recorded in vitro from strips cut from the capsules of mesenteric lymph nodes of sheep. 2. One minute periods of field stimulation at frequencies of 1, 2, 4, 8 and 16 Hz (pulse duration, 0.3 ms) elicited tonic contractions of increasing force and duration. The stimulus frequency-response relationship began to flatten out at frequencies greater than 4 Hz, where the response was already 72% of that at 16 Hz. 3. The response to field stimulation was abolished by tetrodotoxin (1 microM). 4. Phentolamine, rauwolscine and prazosin (all 1 microM) reduced the response to field stimulation, while desipramine (1 microM), potentiated it. 5. Atropine (1 microM) was without effect on the response. 6. These results suggest that sheep mesenteric lymph node capsules have a noradrenergic innervation which modulates their tone via an action on alpha-adrenoceptors.