The histological response of the regional testicular lymph node has been studied at intervals from 1-12 weeks after unilateral vasectomy in rats. The criteria of a humoral immune response were increase in volume of the node, of number and size of germinal centres and of thickness and cellularity of medullary cords. Histological evidence of an immune response was delayed and variable, in conformity with serological data. Variability in the lymphatic drainage of the testis and epididymis has not been finally excluded as a cause of this variation in response. Variability of response did not seem to correlate with variation in site and size of the sperm granuloma, which is thought to be the principal site of leakage of sperm antigens.