Forty-three guinea pigs presented as part of a screening programme were divided into six categories on the basis of age and reproductive history. Three age groupings were used: less than one year, one to two years, and over two years. Each age group was further divided into two categories: animals which had had single or multiple pregnancies, and animals which had never bred. Each animal received a clinical examination and was scanned using B-mode ultrasound for the presence of ovarian cysts. Possible relationships between reproductive history and the prevalence of cysts were investigated using Mantel-Haenzel analysis, and between prevalence, cyst size and age using linear regression analysis. Two out of the 43 animals (4.7 per cent) showed symmetrical alopecia. No statistically significant correlation between reproductive history and the prevalence of cysts was detected at the 95 per cent confidence level. A statistically significant relationship was found, however, between cyst size and age (P<0.01) and between cyst prevalence and age (P<0.02).