Cholecystokinin (CCK) is known to have a short biological half-life. In order to prolong the half-life and create a new investigative tool, we previously PEGylated the peptide, yielding PEG-CCK(9), and demonstrated that it had a dose-dependent prolonged anorectic effect. The aim of this study was to investigate whether PEG-CCK(9) reduces food intake by inducing satiation or by abnormal physiological effects, such as pain, malaise, or nausea. An observational study was performed to examine the effects of different doses of PEG-CCK(9) (1, 2, 4, 8, or 16 microg kg(-1)) on feeding and other behaviors. The behavioral sequence associated with satiety (BSS), i.e. the orderly progression from eating, through grooming and activity, to resting, was analyzed. From the lowest dose tested (1 microg kg(-1)), PEG-CCK(9) caused a dose-dependent reduction in food intake due to a dose-related reduction in both the duration and frequency of eating and a dose-dependent increase in duration of rest. A dose-dependent acceleration in the temporal profile of the BSS was observed, while the normal structure of feeding behaviors was well preserved, except at the dose of 16 microg kg(-1) of PEG-CCK(9), at which a decrease in eating rate and grooming behavior was observed, together with the occurrence of a significant number of abdominal cramps. These findings suggest that the hypophagic response to PEG-CCK(9) is mainly induced by natural mechanisms of satiety, although abnormal physiological effects, such as abdominal cramps, might reinforce the food inhibitory effect, especially at high doses of PEG-CCK(9) (>8 microg kg(-1)).