In mammals, the concentration of testosterone in blood serum typically becomes reduced with aging. We have shown that, in rats, this results from reduced testosterone production by individual Leydig cells. To gain an understanding of the mechanisms by which Leydig cell steroidogenic function changes with aging, we compared the gene expression profiles in Leydig cells isolated from young and old Brown Norway rats, focusing on possible changes in genes involved in, or associated with, steroidogenesis. We also explored the effects of caloric restriction, an intervention shown to delay or inhibit age-associated pathologic and biologic changes in a number of systems and organisms, on the expression of these genes. Of the total of 1176 genes arrayed on each of the five membranes per group that we used, about 500 (45%) were detectable in Leydig cells isolated from young and old rat testes, with the same genes detectable in cells from both ages. An ANOVA model was applied to statistically analyze the 500 detectable genes in the four treatment groups. The primary criterion by which interesting genes were selected was a P value of </=0.05. With aging, a large number of genes were seen with fold changes of >1.5. However, when P</=0.05 was used as the selection criterion, only 45 genes were seen to change significantly with age. Most, but not all, of these genes also had fold changes of >1.5. Among the 500 genes, 17 were found to decrease in expression with aging, the most prominent among which were genes involved in steroidogenesis and stress response/free radical scavenging. Long-term caloric restriction had little effect on the 45 genes altered by age. In contrast to the effects of caloric restriction as seen in many other cell types and systems, the vast majority (44/45) of the Leydig cell genes that changed with age were not 'rescued' by this intervention.