Male adult albino rats living from the 6th week of their lives under conditions of artificial light-dark cycle (16 hrs light, 8 hrs dark) were used to study possible relationships between circadian sensitivity cycles and synchronization of dynamics and the frequency variability of periodicities of reaction times of conditional-reflectory processes with wavelengths in the minute range. The experiments have revealed differences at the time of the activity maximum and minimum of a circadian cycle in the studied parameters of the minute rhythm (mean level of oscillation, mean amplitude, length of period and coupling). In the dark phase (activity maximum) there prevailed an increased level of central-nervous excitation and a tendency towards a higher frequency and diminished coupling of minute periods. In the light phase (activity minumum) the animals showed a balanced level of excitation, reduced frequency and a rigid coupling of periodicities of conditional-reflectory events involved in the process. Relations are shown to exist between central-nervous processes and circadian sensitivity cycles. For the experimental practice it is concluded that the different times of the day should be accounted for in assessing the criteria of minute rhythm and in conditional-reflectory studies in pharmacology and toxicology.