The ultrastructure of peroxisomes in partially differentiated cells of the mouse preputial gland was investigated using serial thin sections and three-dimensional reconstruction as well as the alkaline diaminobenzidine technique for visualization of the peroxidatic activity of catalase. An analysis of serial sections indicates that the different types of intensely stained peroxisomal profiles, classified according to their shape, represent random planes through highly complex peroxisomes. These organelles exceed 4 micron in length and exhibit a focal heterogeneity with respect to their size, shape and enzyme distribution. The graphical three-dimensional reconstruction demonstrates that the most intricate peroxisomes are characterized by tortuous, elongate, and branched tubular segments of varying diameter equipped with enlarged terminal hollow-spherical structures which engulf areas of cytoplasm. A close spatial relationship is established between adjacent peroxisomes and peroxisomes and mitochondria, the latter two of which synchronously develop into highly complex structures. A close association is also observed between peroxisomes and the endoplasmic reticulum, whereby membrane continuities between the two compartments cannot be demonstrated. These observations are inconsistent with traditional concepts concerning peroxisomal shape and size, the number per cell, as well as their biogenesis from the endoplasmic reticulum. The functional significance of individual highly complex peroxisomes and their assemblage forming an extensive net-like membraneous system throughout the cell is discussed with respect to intracellular energy transport and transmembrane electron exchange.