The goal of this study was to evaluate the effects of maternal malnutrition during lactation on serum levels of testosterone and estradiol, testicular testosterone concentration, aromatase, testicular androgen (AR) and estrogen α (ERα) receptors expression in the pups at weaning. From parturition until weaning, Wistar rats were separated into three groups: (C) control group, with free access to a standard laboratory diet containing 23% protein; protein-energy restricted (PER) group, with free access to an isoenergy and protein-restricted diet containing 8% protein; and energy-restricted (ER) group, receiving standard laboratory diet in restricted quantities, which were calculated according to the mean ingestion of the PER group. All pups were killed at weaning, corresponding to 21 days post partum. Compared with the C group, body weights (C = 48 ± 2.3 g; PER = 20 ± 1.3 g; ER = 25.4 ± 0.9 g; P < 0.01) and testicular weights (C = 0.15 ± 0.02 g, PER = 0.05 ± 0.01 g, ER = 0.06 ± 0.02 g, P < 0.001) of both PER and ER groups were lower. However, there was no significant difference in the testicular/body weight ratio in PER and ER groups compared with the C group. The testosterone serum concentration (ng/ml) was significantly higher in the PER group compared with ER and C groups (C = 0.09 ± 0.012; PER = 0.45 ± 0.04; ER = 0.15 ± 0.03, P < 0.01). Testicular testosterone concentration (C = 2.1 ± 0.43; PER = 6.5 ± 0.7; ER = 13 ± 2.3, P < 0.01) was increased in treated groups when compared with controls. The estradiol serum concentration (pg/ml) was lower in both dietary groups (C = 74 ± 4.6; PER = 49 ± 3.2; ER = 60 ± 5.5, P < 0.01). The amounts of aromatase mRNA and ERα transcripts were significantly lower (P < 0.05) in PER and ER groups; conversely AR (both mRNA and protein) was significantly enhanced (P < 0.05) in treated animals. The nutritional state in early phases of development is important since we have demonstrated here that the maternal malnutrition during lactation leads to alterations in estradiol and testosterone serum concentrations, testicular testosterone concentration, AR and ERα expression together with a decrease of aromatase expression. All together, these changes of steroid status may be deleterious for future germ cell development and reproductive function of these male pups submitted to early malnutrition.