There is inadequate basic information available on the reproductive efficiency of peanut (Arachis hypogaea L.) cultivars and a need to determine the sources of yield improvement made in peanut over the past four decades. Therefore, a study of the reproductive efficiency (RE) of 14 virginia market-type peanut cultivars was conducted using field experiments at the Tidewater Research Center, Suffolk, Virginia, in 1983 and 1984. The 14 cultivars vary in maturity from early to late, in release dates from 1944-1981, in breeding method of development from selection within an existing cultivar to hybridization followed by selection, and in growth habit from erect to spreading. The traits studied included flower total (FT), mature pod total (MPT), seed total (ST), pod total (PDT), peg total (PGT), immature pod total (IMPDT), mature pod dry weight (MPDW), plant dry weight (PLDW), vine weight (VW) and seed weight (SW). The five methods used to measure RE were (1) MPDW/PLDW (Harvest Index), (2) MPT/FT, (3) PDT/FT, (4) PGT + PDT/FT and (5) ST/(2*FT). The results indicate that the five most recently released cultivars produced more plowers, 10% more mature pods, 9.7% more mature seeds, and 11% more plant dry weight than the nine previously released cultivars. Using Harvest Index (HI) there was a 4% increase in RE for more recent vs. older cultivars, and about an 8% higher RE for early maturing vs. later maturing cultivars. Also, there was a 3% increase in RE for erect vs. similar advantage for hybrids spreading cultivars and a vs. pure lines. A slight decrease in RE for cultivars released since 1970 vs. those released earlier was observed using Methods 2, 3, 4 and 5. In addition, there was a 4-10% advantage in RE for the two earlier maturing classes over the medium or late classes using the same methods and also a 6% increase in RE for erect vs. spreading cultivars. The growth habit of the 14 cultivars was a very important determining factor for all trait differences. HI appears to be the single best measure of RE in peanuts; however, at least one other method should also be used for obtaining the truest estimate of the RE of a cultivar. It appears that the total peanut cultivar is early maturing and erect with a high RE. This study shows that plant breeders have increased yield in peanuts by (a) increasing apparently the total number of flowers, (b) increasing the HI and (c) increasing reproductive efficiency by increasing the proportion of flowers that form mature pods. Any further increase in yield must combine these three methods of increasing yield as well as overcoming any limiting factors, such as photosynthetic capacity, in order to surpass the present yield plateau. / M.S.