Nitric oxide (NO) is historically well known as a toxic gas but now recognized as a physiological cellular mediator acting at very low concentrations. It is biosynthesized within the body, and modulates many signal transduction processes. For investigation of the functions of this gaseous mediator, it is necessary to use chemical donors that release NO specifically, and it is highly advantageous if the release can be made with precise spatiotemporal control. For this purpose, we have developed caged NO (photocontrollable NO-releasing compounds) with unique releasing mechanisms. One employs the photoinduced rearrangement of an arylnitro group and subsequent release of NO, and another uses photoinduced electron transfer to release NO. One of our caged NO was confirmed to induce a NO-dependent cellular response in vivo under photocontrol. Photocontrollable NO releasers are expected to become indispensable tools for physiological experiments, and are also potential therapeutic agents for photodynamic therapy.