The morphology of apocrine sweat glands in two breeds of cattle, the White Fulani and Friesian was studied histologically using biopsy midside skin samples obtained from the animal at 6.00 a.m. (period 1), 2.30 p.m. (period 2) and 4.00 p.m. (period 3), respectively. Sweat glands with three morphologic types of epithelium were observed. Type 1 consisted of flattened low cüboidal epithelium. Type 2 had tall cuboidal epithelium with cell apical borders slightly indented and the gland lumen narrower than in type 1. A third group of glands had tall columnar epithelial cells (type 3) with bleb-like protuberances on their luminal surfaces. There were also corresponding changes in the morphology of myoepithelial cells from an elongated form in type 1 to a contracted form in types 2 and 3. The morphology of secretory cell surfaces was found to vary with the sampling period; a greater percentage of type 1 glands was seen in period 1 and a preponderance of types 2 and 3 glands (in varying proportions) in periods 2 and 3. Breed differences in the distribution of type of epithelium at various sampling periods were observed. These results are discussed in relation to the functional activity pattern of the sweat glands on stimulation by heat