Many sensor networking applications such as surveillance and environmental monitoring are time-sensitive in nature. To support such applications, we design and implement Nano-RK, a reservation-based real-time operating system (RTOS) with multi-hop networking support for use in wireless sensor networks. We support fixed-priority preemptive multitasking for guaranteeing that task deadlines are met, along with support for CPU and network bandwidth reservations. Tasks can specify their resource demands and the operating system provides timely, guaranteed and controlled access to CPU cycles and network packets in resource-constrained embedded sensor environments. We also introduce the concept of virtual energy reservations that allows the OS to enforce energy budgets associated with a sensing task by controlling resource accesses. A lightweight wireless networking stack supports packet forwarding, routing and TDMA-based network scheduling. Nano-RK has been implemented on the Atmel ATMEGA128 processor with the Chipcon CC2420 802.15.4 transceiver chip. Our results show that a light-weight embedded resource kernel with rich functionality and timing support is practical and constitutes a simple and alternative paradigm for supporting distributed sensing tasks.