A toad (Bufo japonicus) prolactin cDNA was specifically amplified from cDNAs constructed from the total RNA of adenohypophyses, employing the DNA polymerase chain reaction. Sequencing analysis revealed that the cDNA clone thus obtained was 602 bp in length, and encoded the C-terminal 134 amino acid residues of the toad prolactin molecule. The length of the toad prolactin mRNA was estimated to be about 1·0 kb by Northern blot analysis. The partial amino acid sequence deduced from the nucleotide sequence showed the following homologies between toad prolactin and the prolactins of other vertebrates: 69% with man, 80% with chicken, 81% with sea turtle, 91% with bullfrog and 38% with salmon. Using the cDNA as a probe, developmental and seasonal changes in prolactin mRNA levels in the pituitaries of toads were studied. Prolactin mRNA in the pituitary rose as metamorphosis progressed and declined at the end of metamorphosis. During the breeding season the pituitary content of prolactin mRNA was relatively high. This finding suggests that the increases in plasma and pituitary prolactin levels in larvae at metamorphic climax and in adults that remain in or migrate into water, as reported previously, accompany the increase in prolactin synthesis.