Northern grasshopper mice (Onychomys leucogaster) were sampled periodically from June 1977 to November 1986 in the mixed-grass prairie region of north-central Kansas. Necropsies were performed on 175 mice to assess number of pregnancies and litter size for females and testis size for males. Fourteen of 87 females were pregnant with the earliest and latest dates of captures of pregnant females on 22 March and 18 August, respectively. The proportion of pregnant females peaked during May–June. The average number of fetuses was 4.0 (range: 2 to 5), and number of fetuses was correlated positively with body length of the mother. Testes of large males (≥110 mm) increased in size from January to August, and then rapidly regressed through October, whereas smaller males either exhibited a high degree of variability in testes size or had relatively small testes throughout this period. Temporal patterns for pregnancies and testis size suggested that the reproductive cycle for northern grasshopper mice in north-central Kansas was unimodal, and reproductive activity was greatest in late spring to mid-summer.