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The pattern of plasma testosterone was observed during the first 14 months of age in a group of Friesian bull calves reared for meat production. Testosterone was determined by radioimmunoassay in samples collected fortnightly and the data were analyzed by use of polynomial regression and Fourier's transform. Testosterone increased gradually between 4 and 5.5 months, during the period of puberty. After 6.5 months of age plasma testostestone oscillated between about 200 and 400 ng/100 ml at intervals of about 2.5 months.