Cellular aging is accompanied by a reduction in proliferative activity and changes in gene expression. To further elucidate the mRNA phenotype of aging fibroblasts we have monitored the expression of an array of genes implicated in regulating cell-cycle progression. Fourteen genes, including 3 cyclin-dependent kinase (CDK) inhibitors (p16INK4, p21SDI/CIP/WAF and p27KIP), 5 cyclins, 4 CDKs, Cdi-1, and PCNA were tested in four primary fibroblast strains. Relative mRNA expression levels were assessed using a rapid and sensitive Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) assay called the "Primer-dropping" method. p16INK4, a specific inhibitor of the cyclin D-associated kinases CDK4 and CDK6, was, in addition to p21 and cyclin D1, overexpressed in higher passage cells, while the abundance of the D-type kinase mRNAs remained relatively constant. Levels of cyclin H, a component of the CDK-activating kinase (CAK) were markedly reduced in all strains examined, suggesting that the activity of target cyclin/CDK complexes may not be activated in aging cells. These results corroborate and extend previous observations demonstrating elevated expression of specific cell cycle genes in higher passage cells and suggest that overexpression of the CDK-inhibitors p16INK4 and p21SDI/CIP/WAF, but not p27KIP, may contribute to lower proliferative activity of senescing primary fibroblasts.