Maternal undernutrition decreases sperm production in male offspring, possibly through insulin-like growth factor (IGF-I). To test this hypothesis, we fed pregnant Wistar rats ad libitum with a standard diet (CONTROL) or fed 50% of CONTROL intake, either throughout pregnancy (UNP), lactation (UNL, or both (UNPL). After weaning, male offspring (n = 10 per treatment) were fed a standard diet until postnatal day 160, when testes process for histological and molecular analyses. IGF-I immunostaining area and intensity in the testis were greater (P = 0.003) in the UNPL group compared to CONTROL, but lower in the UNP group (P < 0.0001). Levels of IGF-I receptor transcript were lower in the UNPL and UNL groups, compared to CONTROL. There were more Ki-67-positive germ and Sertoli cells, in all underfed groups than in CONTROL. Compared to CONTROL, frequency of spermatogenic cycle stage VII was lower in all underfed groups, and seminiferous tubule diameter was smaller in UNP and UNPL. Plasma FSH concentrations were greater in UNP male offspring compared to all groups (P = 0.05), whereas inhibin B concentrations were greater in UNP (P = 0.01) and UNL (P = 0.003) than in CONTROL or UNPL. Thus, prenatal undernutrition leads to a decrease in testicular IGF-I levels, whereas of pre- and postnatal undernutrition increased testicular IGF-I levels and decreased amounts of IGF-I receptor mRNA in adult offspring. We conclude that maternal undernutrition during pregnancy and lactation leads to long-lasting effects on adult male offspring testicular morphology, spermatogenesis, and IGF-I testicular system.