Dexamethasone regulation of PTHrP expression has been studied in an epidermal squamous cancer cell line COLO 16, which secretes immunoreactive PTHrP into conditioned medium. Dexamethasone was found to suppress PTHrP expression in a time- and dose-dependent manner, which was reversible upon removal of dexamethasone. The half-maximal effective concentration of dexamethasone was 1 nM and an effect of dexamethasone on PTHrP mRNA was first observed after 2 h of treatment, with maximal inhibition by 6 h. Dexamethasone action on PTHrP expression was steroid specific since progestin, 5α-dihydroxy-testosterone and oestrogen did not regulate PTHrP expression in COLO 16 cells. The gluocorticoid/progesterone receptor antagonist RU486 inhibited the dexamethasone effect, indicating glucocorticoid receptor-mediated regulation of PTHrP expression. The half-life of PTHrP mRNA in COLO 16 cells was approximately 120 min and was not altered by treatment of cells with dexamethasone. Nuclear run-on assays revealed that dexamethasone reduced PTHrP gene transcription in COLO 16 cells. Transient transfection assays with a series of reporter gene constructs encompassing 3.5 kb of the 5' end of the PTHrP gene failed to identify a region of the gene responsible for glucocorticoid down-regulation. PCR of reverse-transcribed RNA from COLO 16 cells revealed that dexamethasone down-regulated transcripts driven from all three promoters (i.e., the TATA promoters 5' to exons I and IV and the GC-rich promoter 5' to exon III) of the human PTHrP gene. Key words: Dexamethasone; Parathyroid hormone-related protein; Gene regulation.