The current study examined the effectiveness of an experimental functional analysis for assessing the functions of emergent vocal-verbal behavior in children with developmental disabilities. Experiment 1 consisted of a systematic replication of Lerman et al. (2005). Participants were 3 children with developmental disabilities, between the ages of 2 and 5 years, with limited vocal-verbal repertoires, who could emit at least two clear vocalizations. Results indicated that for all participants, targeted vocalizations functioned as mands, tacts, or both. The purpose of Experiment 2 was to address some of the limitations of Experiment 1 while using a different experimental design. Results of Experiment 2 suggested that participants' targeted vocalizations served as tacts, echoics, or both. Results are discussed in terms of the utility of this methodology for the selection and development of effective language interventions as well as implications for our current knowledge of verbal behavior. (Contains 1 table and 2 figures.)