The present study was undertaken to determine whether the capacity for 3,5,3′,5′-tetraiodothyronine (T4) conversion into 3,5,3′-tri-iodothyronine (T3), as measured by the activity of thyroxine type II 5′-monodeiodinase (T4-5′D), was altered in the brains of young Zucker fa/fa rats during the period of intense maturation of the central nervous system (i.e. from 10 to 20 days of life). From 7 to 14 days of age, no difference in brain T4-5′D activity could be detected between lean and pre-obese rats; serum free T4 was not affected by the fa gene. During the suckling to weaning transition, T4-5′D reached a plateau in brains of lean rats, while it increased by 50% in brains of pre-obese rats; concurrently, serum free T4 increased in Fa/fa rats, whereas it did not change in fa/fa rats. The increased capacity for conversion of T4 into T3 observed in brains of pre-obese compared with lean rats could not be ascribed to a variation in the amount of T4-5′D, since Vmax. did not differ between the two genotypes; however, it could be totally accounted for by an increased affinity of the enzyme for T4. This change may represent an adaptive response to low serum free T4 in order to maintain the cerebral T3 concentration in pre-obese rats. These results show that the alteration in T4 metabolism in brains of fa/fa rats is not an early event and thus cannot interfere with maturation of the central nervous system. However, the decreased serum free T4 which was observed in pre-obese rats after suckling might play a secondary role in development of this genetic obesity.