After pulsed low dose rate irradiation the activities of deoxycytidine kinase and thymidine kinase 1 and 2 were increased 1.5-2-fold 6 h after treatment. Twenty-four hours after treatment the activities of these enzymes had returned to control levels. We presume that the increase of enzyme activities is part of an adaptive response to irradiation and that this increase could be an explanation for the increased survival in the initial part of the SW-1573 cell survival curve. The observation that not only S-phase specific thymidine kinase 1 but also mitochondrial thymidine kinase 2 increases, implies that both these enzymes play a role in an adaptive response of cells to irradiation.