Modifications have been made in the prototype NBS portable sampler to overcome several design flaws. The modifications included replacement of the case, redesign of the filter holder, and addition of a motor control circuit. Results obtained in a brief performance evaluation indicate that the modified sampler yields PM10 data comparable to that obtained with a conventional ambient-type dichotomous sampler. The battery pack (six 1.25V Ni-Cd D cells) used with the modified NBS sampler provides at least 24 hours of operation at a flow rate of 6 1pm. The constant current motor control circuit added to the sampler provides a very uniform flow rate (within + or - 5%) over a 24-hour sampling period. The circuit also automatically decouples the motor when the battery output drops below a voltage that delivers a flow rate of 6 1pm. The performance of an indoor air sampler developed by Harvard University was also evaluated in this study.