Ovulation and pseudopregnancy (psp) can be induced by cervical stimulation in persistent estrous rats maintained in a constant lighting condition (LL-psp rats). Different from the pattern of prolactin (PRL) secretion in usual psp rats maintained in a daily light-dark cycle (LD-psp rats), PRL secretion in these rats during the psp period does not occur in the form of two daily surges. The object of this study was to investigate the role of the external LD cycle for the expression of PRL surges. A large amount of PRL was released in LL-psp rats immediately after cervical stimulation, which is in contrast to LD-psp rats whose first secretion of PRL after cervical stimulation is known to depend on the time of day rather than the time of stimulation. The effect of the change in the lighting condition on the pattern of PRL secretion was investigated in LD- and LL-psp rats. Two surges of PRL were induced in LL-psp rats on Day 5 (Day 0 = day of cervical stimulation), if they were moved from LL to LD on Day 0. Conversely, PRL surges diminished in LD-psp rats on Day 5 if they were moved from LD to LL on Day 0. When the change in lighting conditions was made on Day 5, different results were obtained: In LL-psp rats two daily surges were not established on Day 8 in spite of an application of LD condition from Day 5. In LD-psp rats the two surges were still observed on Day 8, though other time of occurrence was shifted. These results suggest that there is a critical period at and shortly after the time of cervical stimulation during which external LD cycle can work as an entraining agent for the expression of PRL surges. Once PRL secretion is coupled to the external LD cycle, the oscillation continues even in LL condition in a free-running manner. Exposure to LL condition during this critical period inhibits the establishment of two daily PRL surges throughout a psp period.