A study was conducted to determine the effects of 1, 5, 10, and 50 conditioni~ trials per session on classical conditioning of the rabbit's nictitating membrane response. It was found that single-trial sessions produced more rapid increases in CR frequence across trials and required fewer trials to attain criterial performance than multiple-trial sessions. The finding of a superiority of single-trial sessions on CR acquisition was discussed with regard to traditional accounts of trial distribution effects on learning (Le., reactive inhibition, stimulus sampling, and consolidation hypothesis), as well as with regard to stimulus variability accounts. Moreover, consideration was given to the issue of retention as it bears upon time-dependent acquisition processes.