ABSTRACT The structural variations which occur in the dendritic spines of pyramidal neurons of the somatic sensory cortex of the cat are described, particular attention being paid to spines attached to different parts of the dendritic tree. Spines may be recognized particularly by the absence of neurotubules and the common presence of a spine apparatus, and they can be considered as pednnculated or sessile, depending upon the presence or absence of a narrow pedicle. Within these 2 categories spines may be rounded, cup-like or prismatic, can be large or small, and may show various degrees of inversion of the surface receiving a synaptic contact. While spines of every size and shape may be attached to dendrites of all diameters, there is a definite tendency for the largest spines to occur on the smallest dendrites and vice versa. Furthermore, the smallest dendrites possess the spines with the longest pedicles. Every dendritic spine receives 1 axon terminal containing spherical synaptic vesicles and ending in an asymmetrical synaptic contact. In addition, 10–20 % of the spines receive a second terminal, which in some cases may contain spherical vesicles and terminate asymmetrically but in others contains small, flattened or pleomorphic vesicles and ends in a symmetrical contact. The additional terminal may end on the pedicle of the spine or on the parent dendrite near the attachment of the pedicle. In the latter site, asymmetrical synapses are commonly associated with an additional spine apparatus.