Four experiments point to the existence of a transitional period in neonatal rats for the learning of persistence. Partial reinforcement training with suckling on an anesthetized dam as reward induced greater persistence in extinction of the approach response than did continuous reinforcement in rat pups 14 days old but not in 11-day-old pups. Other aspects of this period of development are discussed.The partial reinforcement extinction effect (FREE) describes the relative persistence-increased resistance to extinction-that results from partial or intermittent reinforcement relative to the continuous reinforcement of an instrumental response. Research in our laboratory has demonstrated the occurrence of the FREE in preweanling rat pups (as young as 17-18 days of age) with dry food as reward and the retention of this learned persistence over a 50-day period ). In more recent experiments we have shown that appetitive instrumental learning and extinction is possible in previsual and preaudile (10-to 11-day-old) rat pups (Amsel, Burdette, & Letz, 1976;Amsel, Letz, & Burdette, 1977). There is, however, no evidence of a FREE in 11-day-old rat pups with nonnutritive suckling as reward (Amsel, Letz, & Burdette, 1977, Experiment 2).Consequently, in the present series of experiments, we attempted to identify a transitional period for the manifestation of the FREE. In Experiment 1, we developed a spaced-trial procedure and investigated the effect of the type of suckling experience on acquisition and extinction of an approach response of 11-and 14-day-old rat pups, hi Experiment 2, schedule of reward and type of suckling experience were manipulated in

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Ingestional aversions were conditioned in 12-and 15-day-old rats by infusing a .5% solution of saccharin into the oral cavity and following this oral infusion by the injection of lithium chloride. At both ages, subjects for which the saccharin exposure was followed by lithium injection within 2-3 min drank less when the saccharin solution was again presented by oral infusion 12 hr later; such suppressions of intake were not observed in subjects that previously received the saccharin and lithium in an unpaired fashion (Experiments 1 and 3). Ingestional aversions were also learned by 12-day-olds when a 30-min interval was introduced between saccharin exposure and lithium toxicosis but not when toxicosis was delayed by 120 min (Experiment 2). In contrast, 15day-olds learned aversions with both the 30-and 120-min-delay intervals (Experiment 3). Despite the absence of long-delay learning in 12 day olds, ingestional aversions conditioned at 12 days of age were retained for 2 wk (Experiment 4). These results provide further evidence of the associative abilities of neonatal rats and illustrate a developmental aspect of long-delay learning.There is considerable contemporary in-conditioning in 3-day-olds (Caldwell & terest in the ontogeny of learning in altricial Werboff, 1962; Thoman, Wetzel, & Levine, mammals, particularly in the rat (see 1968, respectively), instrumental avoidance , for a review), learning in 10-to 16-day-olds (Goldman & The tendency has been to study younger and Toback, 1967; Riccio, Rohrbach, & Hodges, younger animals. Recent investigations 1968), appetitive instrumental learning and have demonstrated learning in preweanling extinction in 10-day-olds (Amsel, Burdette, and infant rats; for example, studies have & Letz, 1976), and discrimination learning shown aversive Pavlovian conditioning in in rats 15-and 7-days old (Bulut & Altman, rats 1-8 hr old and appetitive Pavlovian 1974;Kenny & Blass, 1977).Although short-term (24-hr or less) retention of such learning appears to be virtu-The research was supported by Grants BNS 74-19696 all y P erfect ( Riccio et al " 1968 >' long-term

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