Zebrafish behavioral phenotypes are often evaluated in response to pharmacological modulation by various psychotropic drugs. An important step in this process is the method of drug administration. While the most popular drug administration technique in zebrafish research is by immersion, systemic intraperitoneal injection is another effective alternative. This method is useful for drugs that are difficult to dissolve in water, or which require a better control over the amount of drug delivered to an individual animal. Here we outline a simple protocol for the intraperitoneal injection of drugs in adult zebrafish.