The rate of brain 3H-catechol synthesis was measured in rats killed 10 min after receiving a tracer dose of 3H-tyrosine intraperitoneally. Brain 3H-catechol levels were corrected for the specific activity of the 3H-tyrosine precursor. Both increased linearly during this interval. Among rats killed during the afternoon of proestrus, 3H-catechols accumulated in the brain almost four times as rapidly as during diestrus, and more than twice as rapidly as during estrus. Most of the 3H-catechol molecules present in the brain10 min after 3H-tyrosine administration were 3H-dopamine.