Foetal rats were injected with high doses of hCG or ovine LH (oLH) to determine if LH receptors were down-regulated and steroidogenesis impaired as is seen in the adult animal. Foetuses injected in utero with 52 micrograms/kg hCG had a prompt increase in intratesticular testosterone, demonstrating that the injected substance reached the foetal testis. Pregnant mothers were laparotomized and each foetus in one uterine horn injected with 52 micrograms/kg hCG. Twenty-four hours later, the foetal testes from both uterine horns were collected and free LH receptor content measured. In foetuses injected with hCG, free LH receptors were significantly reduced. LH receptors in foetal testes from the contralateral horn were not significantly different from those in controls. Foetuses injected with 52 micrograms/kg hCG at 17.5 days of gestation showed a delayed response in LH receptor up-regulation when compared with neonatal rats injected with comparable doses of hCG. Injection of foetal and neonatal rats with 400 micrograms/kg oLH showed no evidence of down-regulation, as is seen in the adult. Testes from foetuses injected with 52 micrograms/kg hCG 3 days earlier showed an increase in both basal and hCG-stimulated testosterone production during 3 h of incubation when compared with controls. These results show that the rat foetal testis in vivo does not demonstrate down-regulation of LH receptors or steroidogenic lesions of the testosterone synthetic pathway when exposed to high levels of hCG as does the adult rat testis. These results from the foetal testis in vivo are in agreement with results obtained from the foetal testis in vitro and the neonatal testis in vivo.