Neonatal male rats were treated with monosodium glutamate (MSG) at either 0.5, 1.0, 2.0, 3.0 or 4.0 mg/g body weight on alternate days during the first 9 days of life. As adults, rats were catheterized to obtain unstressed, serial blood samples for the determination of ultradian patterns of circulating growth hormone. In addition, the levels of drug-metabolizing enzymes (i.e. hexobarbital hydroxylase, cytochromes P450 and b5, NADPH-cytochrome P450 reductase and ethoxyresorufin O-deethylase) as well as sex-dependent forms of cytochrome P450 [i.e. male-dependent cytochromes P450 2c (IIC11), 2a (IIIA2) and RLM2 (IIA2) and female-dependent cytochromes P450 2d (IIC12) and 3 (IIA1)] and/or their catalytic activities were measured in the hepatic microsomes of the treated rats. The results demonstrated a dose-dependent, graded response to MSG treatment. As the dose of MSG increased from 0.5 to 4.0 mg, there was a concurrent decline in the amplitudes of the characteristically masculine, episodic bursts of growth hormone, until at the highest dose (4 mg), the pulses were no longer detectable. Associated with this dose-dependent alteration in the ultradian pattern of growth hormone secretion was a measurable change in the activities of the sex-dependent hepatic enzymes. As the pulse heights of the hormone declined to 10-20% of their normal amplitudes, the levels of the male-dependent enzymes (i.e. the drug-metabolizing enzymes, as well as the male forms of cytochrome P450 and their specific steroid hydroxylases) were maintained, and in some cases, exceeded the levels normally found in males. However, as the hormone pulse heights declined, there appeared an accompanying increase in the activities of some of the female-dependent enzymes. Finally, with the loss of all detectable levels of circulating growth hormone, the normal masculine profile of hepatic enzymes was reversed to an apparently normal (with the exception of cytochrome P450 2d) feminine profile. Summarizing, the results indicate that (1) neonatal administration of MSG can produce dose-dependent, graded, long-term developmental defects in the ultradian rhythm of circulating growth hormone and associated sex-dependent hepatic enzymes, and (2) while the male-dependent hepatic enzymes can be maintained at normal or even higher levels in the face of an up to 90% reduction in the pulse heights of plasma growth hormone, the activities of the female-dependent enzymes may begin to increase.