The ribosomal S6 kinase family members RSK2 (RPS6KA3) and RSK4 (RPS6KA6) belong to the group of X chromosomal genes, in which defects cause unspecific mental retardation (MRX) in humans. In this study, we investigated the spatiotemporal expression pattern of these genes during mouse development with emphasis to midgestation stages. Additionally, we analyzed the expression of the phosphoinositide-dependent protein kinase-1 gene, Pdk1 (Pspk1), which is essential for the activation of Rsk family members and thus regulates their function. During midgestation we observed specifically enhanced expression of Rsk2 first in somites, later restricted to the dermatomyotome of the somites, then in the sensory ganglia of cranial nerves and in the dorsal root ganglia of the spinal nerves. High Rsk2 expression in the cranial nerve ganglia persists throughout development and is correlated with Pdk1 expression. In the brain of 2-day-old mice, Pdk1 is expressed in the cortical plate of the cerebral cortex and in the stratum pyramidale of the hippocampus, whereas Rsk2 expression is lower in these structures. For Rsk4 ubiquitous expression at lower levels was observed throughout development.