I . Male guinea-pigs were assigned to four different groups at birth. The heaviest group of animals was severely undernourished from 21 d of age (weight gain: 1.2 g/d v. 10 g/d for normally-fed animals).2. At 35, 45, 55 and 65 d of age in normally-fed animals, and at 45 and 55 d of age in undernourished animals, blood testosterone levels were determined by radioimmunoassay, epididymidal tissue examined for the presence of spermatozoa and testes sectioned for rating of spermatogenesis using light microscopy. 3. Sexual maturity in terms of potential fertility (spermatozoa in the epididymis) was reached by all normally-fed animals between 45 and 55 d of age. High birth-weight animals had a higher incidence of the most advanced stages of spermatogenesis than low birth-weight ones at the various ages studied. In the undernourished animals spermatogenesis was clearly delayed.4. Plasma testosterone concentrations were already in the adult range at 35 d and independent of age and birth-weight. The undernourished animals, however, had very low levels. Maintenance of spermatogenesis seemed compatible with low plasma levels of testosterone.5. It is concluded that the timing of testicular development seems to be determined by the time the animals are born and appears to be unaffected by severe undernutrition from 21 d of age.Investigations using various species have indicated that puberty may start earlier in individuals that are large than in those that are small during growth. Investigations have been made in the rat by Kennedy & Mitra (1963) and in man by Tanner (1962). In the female guinea-pig clear correlations have been established between body-weight at birth and the rate of post-natal growth in body-weight (Lister & McCance, 1965) and with the age at which the vagina first opened (Slob et al. 1973).The present study was undertaken to examine the effects of body-weight at birth, and of undernutrition during growth, upon the time of onset of puberty in male guinea-pigs. Criteria for the onset of puberty needed to be established. Therefore, a mixed longitudinalcross-sectional growth study was carried out, in the course of which animals were killed at regular intervals. Testes and epididymidal tissue were examined for sperm production. Blood was collected for testosterone determinations. Part of this study has been reported previously (Slob et al. 1975). EXPERIMENTAL METHODSAlbino guinea-pigs (30-45 d pregnant) were purchased from a commercial breeder (randombred closed breeding colony; TNO, Zeist, The Netherlands). The breeding schedule was such that several litters were born on the same day. Thirty-eight females gave birth to a total of I 28 live young (seventy-seven males and fifty-one females); thirteen (five males and eight females) were stillborn or died shortly after birth. Within 1 2 h of birth (day of birth designated day 0) the pups were sexed and weighed to the nearest g. Within 2 d of birth several litters were mixed and redistributed such that each new 'litter' contained three or four male and female pups; ...

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Morphological data are reported of 69 rats with prolonged vaginal oestrus following the placement of a basal lesion in the anterior hypothalamus, and of 137 rats with regular cycles which were grouped according to the stage of the vaginal smear cycle (early dioestrus, late dioestrus, pro-oestrus, oestrus). The groups of rats with regular cycles comprised 99 intact rats and 38 rats with hypothalamic lesions. Pituitary and uterine weights fluctuate during the cycle, with highest values at pro-oestrus and oestrus. In rats with prolonged vaginal oestrus, the weight of the uterus equals that of regularly cycling rats in oestrus, whilst the weight of the pituitary gland markedly exceeds the highest value found during the normal cycle. Some of the implications of these findings are discussed. Subsequent papers will contain data on pituitary and serum contents of ICSH and FSH in the various groups of control rats and in rats with prolonged vaginal oestrus.