To better characterize the emergence of spontaneous neuronal activity in the developing hindbrain, spontaneous activity was recorded optically from defined projection neuron populations in isolated preparations of the brain stem of the chicken embryo. Ipsilaterally projecting reticulospinal (RS) neurons and several groups of vestibulo-ocular (VO) neurons were labeled retrogradely with Calcium Green-1 dextran amine, and spontaneous calcium transients were recorded using a CCD camera mounted on a fluorescence microscope. Simultaneous extracellular recordings were made from one of the trigeminal motor nerves (nV) to register the occurrence of spontaneous synchronous bursts of activity. Two types of spontaneous activity were observed: synchronous events (SE), which occurred in register with spontaneous bursts in nV once every few minutes and were TTX-dependent, and asynchronous events (AE) which occurred in the intervals between SE and were TTX-resistant. AE occurred developmentally before SE and were in general smaller and more variable in amplitude than SE. SE appeared at the same stage as nV bursts early on embryonic day 4, first in RS neurons and then in VO neurons. All RS neurons participated equally in SE from the outset, whereas different subpopulations of VO neurons participated differentially, both in terms of the proportion of neurons that exhibited SE, the fidelity with which the SE in individual neurons followed the nV bursts, and the developmental stage at which SE appeared and matured. The results show that spontanous activity is expressed heterogeneously among hindbrain projection neuron populations, suggesting its differential involvement in the formation of different functional neuronal circuits.