Oestrous behaviour and occurrence of ovulation was studied in 13 jenny asses (3-18 years) during a 15-month period. Teasing was performed daily and follicular growth, ovulation and changes of the genitalia were checked every other day by rectal palpation during dioestrus or the anovulatory season or daily during oestrus and when a follicle greater than or equal to 3 cm in diameter was present. Plasma oestrogens and progesterone levels were measured by radioimmunoassay (RIA). Of the 13 jennies, 6 showed a typical seasonal ovarian activity, 1 stopped cycling only at the end of the experiment, 2 ovulated regularly and 4 did not cycle. Mean (+/- s.d.) length of the anovulatory season in the 6 jennies with seasonal cyclic ovarian activity was 166.3 +/- 63.2 days (74-263 days). Irregular oestrous periods were detected during the anovulatory season of these jennies. Irregular signs of oestrus and follicular growth were detected in the 4 jennies which remained acyclic during the whole experiment. Mean (+/- s.d.) length of the ovulatory season was 197.8 +/- 63.4 days (102-291 days). Overall mean +/- s.d. lengths for the oestrous cycle and dioestrous and oestrous periods were 25.9 +/- 2.7 (n = 74), 18.2 +/- 2.3 (n = 70) and 7.9 +/- 2.5 (n = 79) days. Ovulatory oestrous periods tended to be longer at the start and end of the season than during the summer months. Dioestrous length was not affected by season.(ABSTRACT TRUNCATED AT 250 WORDS)