Summary. Oocytes were obtained from patients with tubal infertility at fixed times after the onset of the endogenous LH rise or hCG injection, and were inseminated immediately after recovery or after periods of 4–4½, 5–5½ and 6–6½ h in culture in vitro. Delayed insemination resulted in a marked increase in the proportion of oocytes that were fertilized and developed to normal embryos and maximum rates occurred after 5–5½ h in culture (0–½ h, 26%; 4–4½ h, 50%; 5–5½ h, 89%; 6–6½ h, 69%). The range and mean (± s.d.) intervals from insemination for the pronuclear and early cleavage stages were 27–43 (35·6 ± 4·4) h for 2-cell stages, 36–65 (45·7 ± 8·3) h for 4-cell stages, 45–73 (54·3 ± 12·6) h for 8-cell stages and 68–85 h for the 16-cell stage. In 7/50 patients receiving 1 or 2 embryos at the 2-, 4- and 8-cell stages, fetal development was normal and 2 women had twin pregnancies (36% success compared with 8% for single embryos). All pregnancies were from the groups in which insemination was delayed for 5–6½ h. It is concluded that a short period of culture in vitro may allow the completion of oocyte maturation, and improve the results of in-vitro fertilization.