The study determined whether participation credit under voluntary versus called-on conditions differentially affected participation rate and exam scores in college classes. The study was conducted in 6 sections of an entry-level educational psychology course. Approximately 2/3 of the sample were women, and more than 90% were of traditional college age. The sample as a whole included 156 students, from which we identified high- (n = 38) and low- (n = 41) rate participants on the basis of their baseline participation rate. We computed participation rate for each course unit by dividing the number of comments each student made in that course unit by the total number of class comments in that unit. The participation rates for the sample as a whole did not differ significantly between voluntary (3.60%) and called-on (3.62%) units. In a subsequent comparison of initially high- and low-rate participants, the groups had similar participation rates in the called-on condition: 3.99 for the high group and 3.29 for the low group. In the voluntary condition the high-participation group had greater (p < .01) participation rates (4.51%) than did the low-participation group (2.44%). Standardized exam scores did not differ significantly between voluntary and called-on units for the sample as a whole, but the high-rate participants scored higher (p < .01) on all exams than did the low-rate participants in both participation conditions.