SummaryDietary restriction, adrenaline, hydrocortisone or surgery reduced the rate at which pulmonarily arrested125IUdR‐labelled murine tumour cells were lost within 7 h of intravenous (i.v.) injection. Mice that had been adrenalectomised 10 days previously showed a normal intrapulmonary tumour cell loss rate with further surgery reducing this rate to approximately half that observed in normal mice that had been subjected to surgery. Thus, although it is likely that adrenal hormones play an important role in decreasing the rate of early intrapulmonary tumour cell loss, additional factors must be implicated.Mice subject to dietary restriction, adrenaline, hydrocortisone or surgery had reduced levels ofin vitrogrowth inhibitor(s) in their sera. Despite this, individual surgically treated animals showed no correlation between serumin vitrogrowth inhibitor levels and rate of loss of i.v. injected tumour cells from the lungs. Furthermore, the 24 h pre‐incubation of tumour cells in inhibitor‐rich serum did not influence the subsequent loss rate of such cells following i.v. injection into mice.Electron microscopic studies indicated that dietary restriction, adrenaline and surgery reduced the rate of intravascular tumour cell death. The decreased tumour cell death rate in mice receiving these treatments could not be related, however, to any consistent morphological change in the pulmonary vasculature.The decreased rule of intravascular tumour cell death in treated mice was followed by an increased number of lung tumours with only one of the tumour lines studied, indicating that the intravascular death rate need not be a major determinant of pulmonary tumour incidence.