Horseradish peroxidase-wheatgerm agglutinin was injected subcutaneously into one or more nipples of lactating rats to determine the spinal organization of sensory afferents emanating from the mammary glands. After survival periods of 45-96 h, dorsal root ganglia and segments of the spinal cord and/or medulla oblongata were sectioned and reacted histochemically with tetramethylbenzidine to reveal the transganglionically transported tracer. For each nipple injected, the peroxidase reaction product was found in somata, ranging in diameter from 15 to 60 microns, and fibres in 5-11 contiguous dorsal root ganglia. The number of labelled profiles was highest in the 2-4 central-most ganglia of the series and generally decreased progressively rostrally and caudally. After separate injections into each of the six ipsilateral nipples, labelling occurred in all ipsilateral dorsal root ganglia between the 5th cervical and 6th lumbar spinal segments. Substantial overlap of the spinal projections from adjacent mammary glands was seen, a given dorsal root ganglion innervating 2-3 different glands. Label in the spinal cord was restricted to the medial portion of the superficial dorsal horn. It occurred in what appeared to be terminal fields and fibres essentially in the substantia gelatinosa, but was also seen to extend into the marginal zone and sometimes into deeper regions of the dorsal horn. Label was found in both the gracile and cuneate nuclei of the medulla oblongata, though only occasionally and then only very sparsely. The substantial spread and segmental overlap of labelled mammary afferents, and the fact that most labelled afferents terminated in the dorsal horn, suggest that this spinal region may be an important site for the integration of sensory input from the mammary glands that may play a role in the sensory induction of reflex milk ejection.