Substance P, an 11 amino acid residue vasoactive neurotransmitter peptide, has been found on acute infusion (50 micrograms) into cannulated afferent lymphatics of popliteal lymph nodes of sheep to produce marked elevations in both efferent lymph flow and in the outputs of both blast and small recirculating lymphocytes into popliteal node efferent lymph (chronically cannulated). These elevations were characterized by a delay in the onset of major elevations, a marked prolongation of the elevations and a substantially greater stimulative effect on the output of blast lymphocytes. It is suggested that the number and types of substance P receptors on lymphocytes and in sheep peripheral lymph nodes may be responsible for these observations. Infusion of substance P, known for involvement in pain impulse transmission, was able to briefly overcome anaesthesia-induced depression in lymphocyte traffic. The substance P-induced alterations in lymph flow and lymphocyte traffic in vivo were demonstrated to be due to local rather than systemic effects of substance P.