An in vitro system was used for investigating transport of rapidly labeled RNA from liver nuclei isolated from control and thioacetamide-treated rats. An enhanced transport was observed in preparations from thioacetamide-treated animals. Analysis of transport at various temperatures indicated that it proceeded as an energy-requiring process, even in the absence of an exogenously added energy source, and was related to hydrolysis of high-energy nucleoside triphosphate esters. In control preparations, a nucleoside triphosphatase (NTPase) is present in nuclear envelopes, and this TNPase activity appears to be intimately related to in vitro RNA transport. Nuclear membranes were isolated from purified nuclear preparations taken from control and thioacetamide-treated rats and were examined for NPTase activity. The NTPase activity was significantly greater in the preparations from thioacetamide-treated animals. These observations indicate that an early change associated with carcinogen exposure is an enhanced NTPase activity. This may underlie the enhanced RNA transport observed in vitro.