To investigate the mechanisms regulating the nucleotide usage in mammalian genes, we analyzed the sequences of three physically linked Hsp70 paralogs in human and mouse. We report that the sequences of HSPA1A and HSPA1B genes are almost identical, whereas the HSPA1L gene contains some regions very similar to HSPA1A/B and some regions with much higher divergence. Phylogenetic analysis reveals that gene conversion has homogenized the entire coding regions of HSPA1A/B and several fragments of HSPA1L. The regions undergoing conversion are all very GC rich, contrarily to the regions not subject to conversion. The pattern of nucleotide substitution in mammalian orthologs suggests that the mechanism increasing the GC content is still functioning. To test the possibility that the high GC content facilitates the expression of Hsp70 during heat-shock, we performed in vitro translation experiments. We failed to detect any effect of GC content on the translation efficiency at high temperatures. Taken together, our data strongly support the biased gene conversion hypothesis of GC-content evolution.