The diurnal rhythm of rat liver tyrosine aminotransferase was investigated under conditions allowing enzyme activation. The amount of activated enzyme represented only a small proportion of total enzyme. Therefore, the activation mechanism cannot be responsible for the diurnal variations in overall activity. Instead, the results are compatible with the earlier assumption of a generation of the rhythm by periodicde novosynthesis of enzyme protein. Although not of causal importance for the overall rhythm, activation of tyrosine aminotransferase behaved rhythmically: during the phases of the normal diurnal increase in enzyme activity, a certain proportion of enzyme was activated, whereas during the phases of the diurnal decline no activation was observed under the same experimental conditions. Though appearingly correlated with the diurnal stimulation of enzyme production, activation is not a characteristic forde novosynthesized tyrosine aminotransferase, as demonstrated by experiments with hydrocortisone inductions.