Eight adult male rats were chronically cannulated in the jugular vein and placed individually in a sound-attenuated cubicle. Four of the animals were also implanted with a permanent cannula in the right lateral ventricle of the brain. Each animal was submitted twice to auditory stress at a 24-h interval. Before each stress, the rats were pretreated with either saline or alpha-methyl-p-tyrosine (alpha-MT), the order of administration of the drug and its vehicle being alternated in the eight rats. The injections were made either intravenously or intraventricularly. Auditory stress significantly depressed plasma growth hormone (GH) levels irrespective of the type of pretreatment. Mean plasma GH levels were significantly lower after alpha-MT pretreatment. alpha-MT pretreated animals had higher mean plasma corticosterone (B) levels which remained unchanged during stress. Plasma follicle stimulating hormone (FSH) levels were not modified by stress nor by alpha-MT pretreatment. The intraventricular administration of alpha-MT at a dose (20 mg/kg) which is ineffective by a systemic route produced the same effects on GH and B levels as the intravenous injection (250 mg/kg). These data seem to indicate that auditory stress exerts its inhibitory effect on GH secretion through a noncatecholaminergic pathway. They show, on the other hand, the existence of a central catecholaminergic tonus, stimulatory for GH and inhibitory for B. Statistical analysis was based on a factorial mixed design for repeated measurements after logarithmic transformation of the data. The purpose, advantages and limits of this procedure are presented and discussed.