ABSTRACTSubjective uncertainty was introduced during discrimination classical conditioning of the skin conductance and vasomotor responses. All 48 college student subjects were informed that one 7‐sec colored light wouldalwaysbe followed by a shock (CS+) while a 7‐sec light of another color wouldneverbe followed by a shock (CS−). After half of the conditioning trials, a novel stimulus (NS) was illuminated (a third color) and remained on. One group was previously informed about the NS while the other group was noninformed. Results of between trial verbal reports and postsession interviews indicated that noninformed subjects experienced more general uncertainty and more uncertainty regarding the CS‐US contingency on the postNS trials than did the informed group. On the trial pair following the NS onset, the short latency electrodermal response (1 to 4.5 sec) of the noninformed subjects increased while the informed subjects maintained a stable level of responding. Evaluation of later postNS trials revealed that the long latency response (4.6 to 7.9 sec) of the noninformed subjects decreased more than for the informed subjects. The decrease for the noninformed subjects was greatest to CS+. No group differences in vasomotor activity were elicited by NS onset. It was concluded that momentary increases in orienting and more sustained decreases in preparatory activity may accompany increases in subjective unce