The immune and circulatory system of mosquitoes are functionally integrated. A population of hemocytes, called periostial hemocytes, resides on the surface of the heart. An infection induces the migration of additional hemocytes to the heart, and this migration is driven in part by the immune deficiency pathway (IMD). As this is happening, periostial hemocytes produce nitric oxide and lysozymes, and this decreases the heart rate. In ongoing experiments in Anopheles gambiae, we are assessing whether manipulating the IMD pathway alters the heart contraction rate. Preliminary data suggest that silencing the IMD pathway by RNAi in infected mosquitoes reduces the heart rate, and that this is accompanied by changes in the mRNA abundance of nitric oxide synthase and a lysozyme.