Five experiments were conducted to study genetic influences on growth and early egg production in chickens. Parental lines included White Plymouth Rocks divergently selected 25 generations for high (H) and for low (L) 56-day body weight and a closed population of Jersey Black Giants (JJ). F₁ generation crosses (sire listed first) included HL, LH, JL, and JH, and an F₂ generation from HL x HL matings. Comparisons also include dwarf and nondwarf chickens from the H and L lines. Symbols for dwarfs and nondwarfs within the high line were HD and HN and within the low line were LD and LN. Growth patterns of dwarfs more closely approximated that of the H than that of the L line, regardless of whether measured in a chronological or physiological context. Although body components were usually smaller for dwarfs than their within-line nondwarf counterparts, on a relative body weight basis, dwarfs were more uniform than nondwarfs. Dwarfs were also more uniform than normals for age at first egg. The dw allele reduced yolk formation and ovulation rate, resulting in less erratic ovulation and lowering the incidence of defective eggs. Differences between reciprocal crosses were minor as were recombination effects for the traits measured. There was little, if any, evidence of heterosis for body weight and skeletal and lean traits prior to 57 days of age, at sexual maturity, or 60 days after the onset of lay. When present, heterosis was general for most traits measured with the exception of those associated with adiposity which were population dependent. Modest heterosis was observed for the age of inflection of the growth curve, age at sexual maturity, egg production traits, and body weight change during a 60-day laying period. Hybrid vigor for egg production traits resulted in more normal eggs via regularity of yolk development and synchronization of ovulation. The onset of sexual maturity was accompanied by the achievement of a population-dependent body composition, which was generally independent of age and body weight. Under ad libitum feeding, lean carcass traits appeared to be more critical to the onset of sexual maturity than adiposity, as evidenced by greater differences between mature and immature pullets for the former than for the latter traits. The exception to this pattern was the line selected for low juvenile body weight where adiposity traits appeared to be primary.Ph. D.