To test whether or not the House Sparrow (Passer domesticus) is a determinate or indeterminate layer, 16 experimental nest sites were used following a removal protocol suggested by Haywood (pers. comm.). In six of the 16 experimental nests the females laid in an indeterminate manner, laying 8-18 eggs at daily intervals, or with breaks of one or two days. Analysis of the egg contents showed that fresh mass and dry masses of yolk, albumen and shell tended to increase with position in the laying sequence, with fresh mass, dry albumen mass and total dry mass of eggs 2-6 being significantly less than those of the supernumerary eggs, eggs 7+. Female condition was unaffected by the laying of supernormal-sized clutches. The implications of these results for the proximate determination of clutch size in the House Sparrow were discussed, including a proposed mechanism for clutch size determination in the species.