Lick rates in rats were differentially reinforced , using an operant conditioning procedure in which specific lick rates, when maintained for specific temporal durations were reinforced with the opportunity to run in aã ctivity wheel. This operant conditioning paradigm is referred. to as differential reinforcement of high rates of responding (DRH). Between and within Ss lick rate variability did not exceed one lick for ' all DRH conditions tested in which the time base was 1 sec or le~. Specific momentary lick rates were successfully rem forced for all Ss tested, but DRH conditioning did not p.roduce higher momentary lick rates than the range of lick-rate values normally observed in baseline conditions.All evidence presently available suggests that the licking response in rats occurs at a relatively constant momentary rate. Stellar & Hill (1952), the earliest investigators to explore rate of licking in the rat, found that (a) rats drink at a relatively constant rate or not at all, (b) the average rate of licking is 6-7 laps/sec, and (c) this rate is independent of fluid deprivation levels.Davis & Keehn (1959) further substantiated the co~stancy of the lick rate in the rat by presenting evidence that the average lick rate was constant for saline, sucrose, and saccharin solutions. In 1960, Davis and Arnold concluded that slight intersession and individual differences in lick rate, of the order of about 1 lick/sec , did exist . . Further evidence that lick rates in the rat are largely independent of environmental influence was presented by Schaeffer & Premack (1961). They found that neonatal .rats , raised without an opportunity to drink, upon then first contact with fluids displayed a mean lick rate identical to that of adults. Unlike previous investigators, Schaeffer and Premack showed that careful analysis of momentary lick rate within bursts of over 1 sec duration yielded initial actual rates of licking for some animals as high as 9 .0 licks/sec and actual terminal rates within the same burst as low as 5.2 licks/sec . which averaged to a mean lick rate for the entire lick burst of 6.6 licks/sec. As have other investigators , however , they found that both the mean and modal lick rates for rats were 6·7 licks/sec. Smith (1966). These studies, in addition to revealing that the lick response fatigued , could be classically conditioned , and , once init iated, was reflexive, all reported that lick rate was essentially constant. Furthermore, the reflexive properties of momentary lick rate were not affected by radiation doses 10 times the strength needed to produce aversion to normally preferred saccharin solutions (Schaeffer & Smith, 1966).Recently, Corbit & Luschei (1969) used a computer to measure interlick intervals in milliseconds and found that neither different deprivation schedules, preferred taste stimulation, nor aversive taste stimulation affected the interlick interval. The mean rate of licking , as measured by this highly precise method , equalled 6.5 licks/sec, which compares favorably with the rates f...