The development of somatostatin (SS) neurons and fibers has been examined in the dorsolateral cortex of the mouse mutant reeler. Immunohistochemistry was performed using antisera directed primarily against SS28 or SS28(1-12). In the normal mouse at postnatal day 5 (P5), somatostatin (SS) neurons are concentrated in the ventral half of the cortex, in the developing layers V and VI. In the reeler mutant, SS neurons are scattered throughout the radial extent of the cortex, being concentrated in the dorsal half of the cortex in the polymorphic and large pyramidal cell layers. By P20, when the adult pattern of SS neuron distribution is evident in the normal mouse cortex, the distribution of similar neurons in the reeler appears inverted: immunoreactive neurons are concentrated in the dorsal half of the cortex. Immunoreactive fiber distribution follows a developmental pattern similar to that observed for SS neurons. At P5, SS fibers are most dense in layer I and V-VI of the normal cortex, while in the reeler, fibers are predominant in the polymorphic and large pyramidal cell layers. By P10, many fewer immunoreactive fibers can be detected in either normal or reeler mice than at P5. Nevertheless, while SS fibers in the normal cortex are most dense in layers I and V-VI, the reeler cortex exhibits little laminar heterogeneity in the distribution of these fibers. Thus, the SS fiber distribution appears less organized in the reeler cortex. These results suggest that whatever the nature of the genetic alteration resulting in cortical cellular developmental malposition in the reeler, SS cells and fibers, representing a completely intrinsic neocortical cellular system, behave as do all other cortical elements.