The present study illustrates the use of brief functional analysis probe conditions to verify the results of a descriptive assessment. An initial descriptive assessment of the disruptive behaviour of an 8-year-old student with severe developmental disabilities showed that levels of disruptive behaviour (screaming and throwing equipment) were higher in some lessons than others and suggested that the behaviour might be maintained by escape from task demands. An intervention in which work demands were alternated with 5-minute periods of free activity reduced levels of screaming to under 50%, and of throwing to under 25%, of baseline levels. Brief experimental variations of demand level in some lessons confirmed that levels of disruption were generally higher under high demand conditions. We conclude that brief probes provide a method by which experimental analyses can be conducted in the client's natural environment, reducing the problem of non-occurrence of the target behaviour that can pose problems for analogue assessments and facilitating ongoing assessment during initial intervention. We note also, however, that the consequent reduction in control over establishing operations may reduce the precision of the analysis and that ethical considerations limit the range of behaviours for which the method is appropriate.