The nucleotide (nt) sequence of the cDNA encoding the retinal rod cyclic 3'5'-GMP phosphodiesterase (PDE) alpha and gamma subunits from two strains of dogs-(i) Labrador Retrievers homozygous for autosomally recessively inherited rod-cone degeneration and (ii) the wild-type Beagle-are reported. Cloning of these subunits was accomplished by polymerase chain reaction using retinal cDNA libraries as templates. The nt sequence of alpha PDE predicts a 861-amino-acid polypeptide which is 97.7 per cent and 96.9 per cent identical to the bovine and human counterparts, respectively. PDE gamma encodes an 87-amino-acid polypeptide differing from bovine and murine gamma subunits by only one amino acid. Since no differences were found between these two strains of dogs, the cause of the Labrador Retriever's degeneration remains to be determined.