In order to analyse the relationship between the ovarian response to stimulation in in-vitro fertilization (IVF) treatment cycles and relaxin concentrations during subsequent pregnancies, 31 healthy women pregnant after IVF treatment were studied prospectively. The maximum number of follicles observed from day -4 to day -2 in relation to ovum retrieval and the number of oocytes recovered were recorded. In addition, blood samples were drawn in the follicular phase, the luteal phase, early pregnancy and at gestational weeks 12, 16, 20, 27 and 35 to assess oestradiol, progesterone, human chorionic gonadotrophin and relaxin. The maximum numbers (mean +/- SEM) of follicles observed and oocytes recovered were 9.0 +/- 0.6 and 6.1 +/- 0.5 respectively. The supraphysiological mean relaxin values were strongly correlated to the maximum number of follicles observed (r = 0.72, P < 0.0001) and the number of oocytes recovered (r = 0.64, P < 0.0001), indicating that the source of increased relaxin production during IVF pregnancy might be the ovary. These results are supported by experimental data. In the present study, the occurrence of multiple pregnancy was not associated with higher relaxin concentrations, which is further support for the hypothesis that the ovary is the main source of serum relaxin.