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Abstract Phenotypic sexual dimorphism often involves the hormonal regulation of sex-biased expression for underlying genes. However, it is generally unknown whether the evolution of hormonally mediated sexual dimorphism occurs through upstream changes in tissue sensitivity to hormone signals, downstream changes in responsiveness of target genes, or both. Here, we use comparative transcriptomics to explore these possibilities in 2 species of Sceloporus lizards exhibiting different patterns of sexual dichromatism. Sexually dimorphic S. undulatus develops blue and black ventral coloration in response to testosterone, while sexually monomorphic S. virgatus does not, despite exhibiting similar sex differences in circulating testosterone levels. We administered testosterone implants to juveniles of each species and used RNAseq to quantify gene expression in ventral skin. Transcriptome-wide responses to testosterone were stronger in S. undulatus than in S. virgatus, suggesting species differences in tissue sensitivity to this hormone signal. Species differences in the expression of genes for androgen metabolism and sex hormone-binding globulin were consistent with this idea, but expression of the androgen receptor gene was higher in S. virgatus, complicating this interpretation. Downstream of androgen signaling, we found clear species differences in hormonal responsiveness of genes related to melanin synthesis, which were upregulated by testosterone in S. undulatus, but not in S. virgatus. Collectively, our results indicate that hormonal regulation of melanin synthesis pathways contributes to the development of sexual dimorphism in S. undulatus, and that changes in the hormonal responsiveness of these genes in S. virgatus contribute to the evolutionary loss of ventral coloration.