Many studies support the concept of insulin synthesis in tissues other than the pancreas. Our previous investigations have demonstrated the presence of insulin immunoreactivity in the adrenal medulla of the rat. This immunoreactivity was found to be associated with the chromaffin granule. This study is directed at isolating the messenger ribonucleic acid that encodes for preproinsulin. Reverse transcription coupled with polymerase chain reaction was used. Complementary deoxyribonucleic acid (cDNA) was amplified, extracted and reamplified. It was then subjected to digestion with four different restriction endonuclease enzymes. Its resemblance to the corresponding cDNA that encodes for preproinsulin I in the rat was established. Our results suggest that insulin is synthesized in the rat adrenal gland for autocrine, paracrine, neuromodulation or local physiologic function.