Plasma concentrations for several androgens and estrogens were determined in male domestic pigs from birth to eight months of age. Samples (N = 6) of blood were collected from Yorkshire males at weekly intervals from birth to four weeks, and thereafter at monthly intervals to eight months. Radioimmunoassays were done without extraction from plasma for dehydroepiandrosterone sulphate, androstenedione and estrone sulphate. Other steroids were measured after solid-phase extraction, separate elution of unconjugated and conjugated fractions, and solvolysis of sulpho-conjugated steroids (testosterone, 5α-androstane-3β, 1 7β-diol, epiandrosterone, 19-nortestosterone and estradiol-177β). All steroids showed a peak in plasma levels at 2–4 weeks after birth. Concentrations remained low from 2–5 months and rose markedly thereafter. Most steroids were present in much greater quantities as sulpho-conjugated compounds. Concentrations of testosterone sulphate and testosterone were similar (9.4 μmol/l) at three weeks but the sulphated form predominated after six months of age. This study shows that during postnatal development the testes of the domestic pig are remarkably active in steroidogenesis with a peak at 2–4 weeks after birth. Also, the range of steroid products seen at this stage is comparable to that shown by the mature boar.