Rats and fish were trained to give a free operant response under partial versus consistent reinforcement and high versus low effort. Resistance to extinction was then measured in terms of number of responses to criterion (R), time to criterion (T), and rate to criterion (R/T). Effort failed to affect any of these measures in either species. In rats partial reinforcement produced greater resistance to extinction than consistent reinforcement on R and T, but not on R/T, whereas partially reinforced fish failed to differ from those consistently reinforced on R and T bur exceeded the consistently reinforced fish on R/T. The R and T measures were correlated in both groups of fish but only in the consistently reinforced rats. Partially reinforced fish were less variable than consistently reinforced fish on the T measure. It is suggested that the principal effect of partial reinforcement in rats is on mean time to extinction, but the effect in fish is on variability of time to extinction.
