Parental care is thought to increase offspring success but entails a cost in terms of a decrease in the parent's ability for subsequent reproduction. Females of most bees and wasps show extensive parental care. Thus, they are ideal organisms to study the expenditure for offspring production and the resulting costs of reproduction. We examined parental behaviour in the solitary bee Osmia rufa (Linné 1758) (Hymenoptera Megachilidae) by use of a new method that allows observation of within nest behaviour and quantification of most aspects of nest construction and provisioning. The method does not cause disturbance of provisioning females and does not increase immature mortality. To characterise maternal expenditure we recorded the number of foraging trips and the time spent for one brood cell, the duration of single foraging trips to gather provisions and loam (for brood cell partitions), and the amount of provisions and loam gathered per trip. We predicted that, due to the large expenditure for provisioning, females would loose weight in the course of the flight season. Alternatively, a weight loss could be caused by decreasing ambient temperatures and a shortage of food resources. Our results show that female O. rufa lost mass towards the end of the flight season. Though ambient temperatures had some effect, the high expenditure for reproduction were probably the major cause of the weight loss. The weight loss was independent of the rate of brood cell construction but was significantly more pronounced in smaller females. Thus, if the weight loss represents a cost of reproduction our results suggest that the magnitude or type of cost might vary among individuals.