Functional interactions between the levels of clock gene expression and adrenal steroidogenesis were studied in human adrenocortical H295R cells. Fluctuations of Bmal1, Clock, Per2 and Cry1 mRNA levels were found in H295R cells treated with forskolin (FSK) in a serum-free condition. The changes of clock gene expression levels were diverged, with Clock mRNA level being significantly higher than Cry1 and Per2 mRNA levels after 12-h stimulation with FSK. After FSK induction, mRNA levels of StAR and CYP11B2 were highest at 12 hours and CYP17 mRNA level reached a peak at 6 hours, but HSD3B1 mRNA level was transiently decreased at 3 hours. The expression levels of Clock mRNA showed a significant positive correlation with StAR among the interrelationships between mRNA levels of key steroidogenic factors and clock genes. Knockdown of Clock gene by siRNA led to a significant reduction of FSK-induced expression of StAR and CYP17 after 12-h treatment with FSK. BMP-6 and activin, which modulate adrenal steroidogenesis, had inhibitory effects on Clock mRNA expression, whereas treatment with follistatin, a binding protein of activin, increased Clock mRNA levels in the presence of FSK, suggesting an endogenous function of activin in regulation of Clock mRNA expression. Collectively, the results indicated that changes of Clock mRNA expression, being upregulated by FSK and suppressed by BMP-6 and activin, were tightly linked to StAR expression by human adrenocortical cells.