Japanese quail lines were divergently selected for 30 generations for 4-wk BW (HW, LW) or plasma yolk precursor (HP, LP) as measured by total plasma phosphorus (TPP) at the beginning of lay. The HW, LW, HP, and LP lines were initiated from a randombred control line (R1). In the ninth generation of selection in the HW line, sublines were developed by continuing to select males for increased 4-wk BW whereas females were selected for increased (HW-HP line) or decreased (HW-LP) TPP. Individual selection was used in the HW, LW, HW-HP, and HW-LP lines. In the HP and LP lines, females were individually selected whereas males were selected on their full-sisters' performance. Realized heritabilities (h2), based on the linear regression of selection responses (corrected for environmental changes) on accumulated selection differentials (weighted for the number of offspring produced) over generations, declined with selection in the HW, LW, HP, and LP lines. The h2 of 4-wk BW in the HW line was 0.42 during the first 10 generations of selection and then declined to 0.30 during the next 2 generations of selection. In the LW line, realized h2 of 4-wk BW was 0.40, 0.16, and -0.12 in Generations 1 through 10, 11 through 20, and 21 through 30, respectively. The respective h2 for TPP in the HP line were 0.29, 0.17, and 0.14. The h2 of TPP in the LP line was 0.35 during the first 10 generations and essentially zero during the next 20 generations. Four-week BW and TPP were negatively associated in the HW, HP, and LP lines. There was no significant relationship between these two traits in the LW line.