MicrORNAs (miRNAs) are a class of short, evolutionary conserved non-coding RNA molecules, which are shown as the key regulators of many biological functions. External stress can alter miRNA expression levels, thereby changing the expression of mRNA target genes. Here, we show that miR-21 is involved in the regulation of alkalinity tolerance in Nile tilapia. Alkalinity stress results in a marked reduction in miR-21 levels. miR-21 loss of function could affect ion balance regulation, ROS production, and antioxidant enzyme activity in vivo. Moreover, miR-21 knockdown protects cell against alkalinity stress-induced injury in vitro. miR-21 directly regulates VEGFB and VEGFC expression by targeting the 3'-untranslated regions (UTRs) of their mRNAs, and inhibition of miR-21 significantly increases the levels of VEGFB and VEGFC expression in vivo. Taken together, our study reveals that miR-21 knockdown plays a protective role in alkalinity tolerance in tilapia. (C) 2016 Elsevier Inc. All rights reserved.