Purpose: Photoperiod is known to cause physiological changes in seasonal mammals, including body weight, physical activity, and reproductive status. Because cats are seasonal breeders, we recently tested the effects of day length on resting metabolic rate, voluntary physical activity, and food intake. In that study, resting metabolic rate, physical activity, and food intake to maintain body weight were greater in cats exposed to long days vs. short days. Because photoperiod has also been demonstrated to affect adipose tissue gene expression in several species, including dairy cows, sheep, and Siberian hamsters, the objective of this study was to determine the effects of day length on the adipose transcriptome profile of cats as assessed by RNA-seq. Methods: Ten healthy adult neutered male domestic shorthair cats were used in a randomized crossover design study. During two 12-wk periods, cats were exposed to either short days (8 hr light:16 hr dark) or long days (16 hr light:8 hr dark). Cats were fed a commercial diet to maintain baseline body weight. Subcutaneous adipose biopsies were collected at wk 12 of each period for RNA isolation and Illumina sequencing. Results: A total of 578 million sequences (28.9 million/sample) were generated by Illumina sequencing. Using a raw p value of P<0.005, 170 mRNA transcripts were differentially expressed between short day- and long day-housed cats. Of the 170 transcripts highlighted, 25 annotated transcripts were up-regulated, while 116 annotated transcripts were down-regulated by long days. Another 29 un-annotated transcripts (name and function not known) were also different between groups. In general, adipose tissue of long day-housed cats had greater expression of genes involved with cholesterol trafficking, fatty acid synthesis and immune function, and lower expression of genes involved with cell cycle and growth, cell development and structure, and protein processing, when compared to short day-housed cats. Overall design: Subcutaneous adipose tissue mRNA profiles of healthy adult neutered male cats exposed to short days (8 hr light: 16 hr dark) or long days (16 hr light: 8 hr dark) using Illumina sequencing.