EXPRESSION OF HIGH-ACTIVITY OF ORNITHINE DECARBOXYLASE AND OCCURRENCEOF UNUSUAL CHROMOPHOBIC CELLS IN ANTERIOR-PITUITARY GLAND OF A NOVEL GROWTH-RETARDED STRAIN OF MICE, GRM GRM/

Extremely high activity of ornithine decarboxylase (ODC) was detected in the pituitary gland of growth-retarded mice, grm/grm at 2 months af ter birth. The elevated enzyme activity gradually decreased to the con trol level in 14 months after birth. In the pituitary gland of the gro wth-retarded mice, unusual chromophobic cells were also present from t he early stages after birth. The chromophobic cells showed conspicuous proliferations and resulted in a distinct hyperplasia of the tissue a fter 4 months after birth. These findings suggest that ODC is correlat ed to the progressive transformation of pituitary cells into the chrom ophobic cells.