Sparrowhawk nests (395) found in areas centered on the Esk and Annan Valleys in S Scotland [United Kingdom], in 1971-73 were studied. Breeding was earliest and most successful in small valley woods in Eskdale, followed in order in Annandale by small valley woods, hillside forests and the large Ae Forest, which covered several hills. In the valley woods, compared to hill forests: mean laying dates were earlier (in 3 yr), mean clutch and brood sizes were larger (3 yr and 1 yr), bigger proportions of nests produced young (1 yr) and nestlings survived better (1 yr). Most such differences were apparent in all 3 yr, but were statistically significant only as mentioned. They were associated with a known greater abundance of prey in valleys than in hills. The most common causes of nest failure were those that occurred at an early stage. Organo-chlorine levels were significantly higher in eggs from clutches which were broken or failed to hatch after incubation than in eggs from clutches which hatched normally. Levels in eggs from deserted clutches were not significantly higher than in eggs from clutches which hatched normally. Failures from direct human intervention accounted for at least 7% of all nests and 15% of all failures; they were due mainly to gamekeepers shooting breeding birds and were restricted to valley woods. Non-laying (having built a nest) and desertion of eggs were significantly more frequent in hill forests than in valley woods, and accounted for much of the difference in breeding output between these environments. Other types of total failure did not differ greatly between environments and those attributable to organo-chlorine compounds (egg-breakage and embryo deaths) accounted for at least 12% of all nests. Some birds of both sexes bred in their 1st yr and formed a greater proportion of the breeding population in small valley woods than in large hill forests. In valleys, yearling females tended to lay later and smaller clutches than older ones.