Two phenobarbital-inducible forms of cytochrome P-450, P-450(PB-1), and P-450(PB-4), were purified from the liver microsomes of phenobarbital-treated rats and identified with P-450b and P-450e, respectively. It was found, however, that the content of P-450(PB-4) in the liver microsomes of a strain of SD rat, Qdj:SD, was very low even after phenobarbital-induction. The levels of the mRNAs for P-450(PB-1) and P-450(PB-4) were separately determined using Northern blot hybridization with specific oligonucleotide probes. It was found that the level of P-450(PB-4) mRNA in the livers of phenobarbital-treated Qdj:SD rats was much lower than that of phenobarbital-treated Slc:SD rats. Slc:SD rats are widely used in Japanese laboratories. Genetic analysis using the crossbred animals between Qdj:SD and Slc:SD rats showed that the low expression of P-450(PB-4) in Qdj:SD rats is a recessive trait and is caused by a single gene mutation. However, no difference in the 5' flanking region in P-450(PB-4) gene was found between Qdj:SD rats and Slc:SD rats.