Two studies were carried out comparing the effects of clobazam (a 1,5-benzodiazepine), and clonazepam (a 1,4-benzodiazepine), with placebo, in a double-blind crossover design. Ten and nine healthy male volunteers, respectively, participated in each study. Performance on a series of automated psychological tests was assessed on four occasions: prior to any treatment; at the end of the first 2-week treatment period; following a 2-week washout phase, and again at the end of a second 2-week treatment period. Blood samples were collected at the last three occasions for analysis of drug levels. Minimal impairment in cognitive functioning was found with clobazam, only two of the more complex tasks being affected. In contrast, the pattern of findings with clonazepam suggests that it may have adverse affects on a broader range of functions. A relationship was found between performance on three measures and serum level of clobazam, such that response latency decreased with increasing serum level. In contrast, for clonazepam, a significant correlation was found on one task, in the direction of increased response latency with increasing drug level. These findings are discussed in the light of clinical literature about the use and efficacy of these two benzodiazepines, as antiepileptic agents.